Editing Tokugawa Ieyasu (section)

==Ieyasu-Nobunaga Alliance (1562–1582)==
Sometime in the aftermath of the Okehazama battle where Imagawa Yoshimoto was slain, Motoyasu formed the {{ill|Kiyosu Alliance|jp|清洲同盟}} with Oda Nobunaga, daimyo lord of [[Owari Province]] and the head of the Oda clan.<ref>{{Cite journal|author=Masaki Anno |title=鋳物師水野太郎左衛門 |journal=弘前大学教育学部研究紀要クロスロード |issn=1345-675X |publisher=弘前大学教育学部 |date=2002 |issue=6 |pages=1–15 |hdl=10129/2159 |url=https://hdl.handle.net/10129/2159 }}</ref><ref>{{Cite journal|author=Masaki Anno |title=瀬戸宛て信長制札 |journal=弘前大学教育学部紀要 |issn=0439-1713 |publisher=弘前大学教育学部 |date=2004 |issue=91 |pages=15–26 |hdl=10129/2162 |url=https://hdl.handle.net/10129/2162}}</ref>{{efn|Historians cannot reach consensus about the exact date when the alliance was officially proclaimed. Some have said the alliance was established two years after the Okehazama battle, although Ieyasu himself did not attend personally to the [[Kiyosu Castle|Kiyosu castle]], since there are no historical records about his visit during this time.<ref>{{Cite web|title=織田信長と徳川家康が結んだ「清須同盟」は幻だった⁉ 徳川家康の真説 |url=https://www.rekishijin.com/25015|website=歴史人|access-date=2023-02-02}}</ref> Other theories from Akio Hirano deduced the alliance only formally announced in 1573.<ref>{{Cite journal|author=Hirano Akio |title=戦国期徳川氏の政治的立場--織田氏との係わりを通して |journal=国史学 |issn=0386-9156 |publisher=国史学会 |date=1995 |issue=158 |pages=97–128 }}</ref>}}

In 1563, Matsudaira Nobuyasu, the first son of Motoyasu, was married to Oda Nobunaga's daughter [[Tokuhime (Oda)|Tokuhime]].<ref name="Pitelka; Tokugawa" />{{Page needed|date=June 2024}}{{efn|[[Tetsuo Owada]] stated the marriage between Nobuyasu and Tokuhime occurred in 1567, when both was nine years old.<ref name="Tetsuo Owada 2002 24">{{harvtxt|Tetsuo Owada|2002|p=24}}</ref>}} In February, Matsudaira Motoyasu changed his name to Matsudaira '''Ieyasu'''.<ref name="Pitelka; Tokugawa">{{cite book |last=Pitelka |first=Morgan |title=Spectacular Accumulation: Material Culture, Tokugawa Ieyasu, and Samurai Sociability |date=2015 |publisher=University of Hawai'i Press |isbn=9780824851576}}</ref>{{Page needed|date=June 2024}}<ref name="Brinkley; Tokugawa">{{Cite book|last=Brinkley|first=Frank & Kikuchi|url=https://books.google.com/books?id=bzRen1dcdTwC&q=Tokugawa+Ieyasu,February,changed+his+name&pg=PT818|title=A History of the Japanese People From the Earliest Times to the End of the Meiji Era|date=1912|publisher=Library of Alexandria|isbn=978-1-4655-1304-5|language=en}}</ref> Some historians believe that these actions provoked the pro-Imagawa faction, including the Sakurai and Okusa Matsudaira families, which led to the simultaneous uprising against Ieyasu in the following year.{{Sfn|Shiba|2021|p=23 quoting "General Overview: Tokugawa Ieyasu's Movements and Research in the Sengoku and Oda-Momoyama Periods"}}

===Unification of Mikawa===
{{Main|Mikawa province}}

[[File:Azukizaka 1564.JPG|thumb|[[ukiyo-e]] print depicting the [[Battle of Azukizaka (1564)|Battle of Batogahara]] between Tokugawa clan against [[Ikkō-ikki]] movement in 1563–1564.]]

During this period, the Matsudaira clan faced a threat from the [[Ikkō-ikki]] movement, in which peasants banded together with [[Sōhei|militant monks]] under the [[Jōdo Shinshū]] sect, and rejected the traditional feudal social order. Ieyasu undertook several battles to suppress this movement in his territories, including the [[Battle of Azukizaka (1564)]]. 

Some of Ieyasu's vassals were in the Ikkō-ikki ranks, notably[[Honda Masanobu]] and [[Natsume Yoshinobu]], who had deserted him for the Ikkō-ikki rebellion out of religious sympathy.{{sfn|Turnbull |1998 |p=216}} However, many of Ieyasu's core vassals who were also followers of the sect, such as Ishikawa Ienari<ref name="Ishikawa Ienari">{{Cite book|author=Sunao Kawaguchi (川口素生)|title=戦国軍師人名事典|publisher=学習研究社|date=2009|page=302}}</ref> and [[Honda Tadakatsu]], quickly abandoned the Ikkō faith of Jōdo Shinshū and stayed loyal to Ieyasu in order to confront the rebels.<ref name="Turnbull 2012; Honda Tadakatsu">{{harvtxt|Stephen Turnbull |2012 |pp=13, 27, 34}}</ref><ref>{{cite book |author1=Tadachika Kuwata |title=日本の合戦. 第5巻 (織田信長) |trans-title=Battle of Japan Volume 5 (Oda Nobunaga) |date=1965 |publisher=人物往来社 |location=Tokyo |page=207 |url=https://ndlsearch.ndl.go.jp/books/R100000002-I000000867858 |access-date=23 May 2024 |language=Ja}}</ref> 

On January 15, 1564, Ieyasu decided to concentrate his forces to attack and eliminate the Ikkō-ikki from Mikawa. In the [[Battle of Azukizaka (1564)|Battle of Azukizaka]], Ieyasu was fighting on the front lines and was nearly killed when he was struck by several bullets, but survived because they did not penetrate his armor.<ref name="Turnbull 2012; Honda Tadakatsu" /> Both sides were using new gunpowder weapons which the [[Portuguese Empire|Portuguese]] had introduced to Japan 20 years earlier. At the end of battle, the Ikkō-ikki were defeated. By 1565, Ieyasu had become the master of all of Mikawa Province.{{Citation needed|date=May 2024}}

In 1566, as Ieyasu declared his independence from the Imagawa clan, he reformed the order of Mikawa province, starting with the Matsudaira clan, after he pacified [[Mikawa Province|Mikawa]]. This decision was made after he was counseled by his senior vassal [[Sakai Tadatsugu]] to abandon the clan's allegiance to the Imagawa clan.<ref>{{harvtxt|Arthur Lindsay Sadler |2014|p=57}}</ref> He also strengthened his power base by creating a military government system for the Tokugawa clan in Mikawa which was based on his hereditary vassals, the [[Fudai daimyō]]. The system which was called "''Sanbi no gunsei''" (三備の軍制) divided governance into three sections:<ref>{{cite book |author1=Tamotsu Fujino |title=徳川政権と幕閣 |trans-title=Tokugawa government and Bakufu |date=1995 |publisher=新人物往来社 |location=11 |url=https://books.google.com/books?id=SYINAQAAMAAJ |access-date=27 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Tamotsu Fujino |title=徳川幕閣: 武功派と官僚派の抗争 |trans-title=Tokugawa Shogunate: Conflict between the military faction and the bureaucratic faction |date=1967 |publisher=中央公論社 |pages=16, 29 |url=https://books.google.com/books?id=DnJDAAAAYAAJ |access-date=27 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Rizō Takeuchi |title=角川日本地名大辞典: 愛知県 |trans-title= |date=1978 |publisher=Kadokawa Shoten |page=41 |url=https://books.google.com/books?id=ZgMRAQAAMAAJ |access-date=27 May 2024 |language=Ja}}</ref>

# [[Hatamoto]]-Senshi: Ieyasu's direct vassals and personal unit of the army. Their task was to personally protect Ieyasu. The earliest commanders of this unit included Matsudaira Ietada (Tojo), [[Torii Mototada]], [[Honda Tadakatsu]], [[Sakakibara Yasumasa]], [[Ōkubo Tadayo]], Osuga Yasutaka, Uomura Iezumi, and others
# Higashi Mikawa: The eastern Mikawa province army unit, put under the control of Sakai Tadatsugu as overall commander. The commanders of this unit consisted of many Matsudaira clansmen and other hereditary vassals of Tokugawa such as [[Matsudaira Ietada (Fukōzu)]], [[Matsudaira Tadamasa]], [[Matsudaira Ietada (Katahara)]], and others
# Nishi-Mikawa: The western Mikawa province army unit, put under the ''de jure'' control of Ishikawa Ienari (''de facto'' control lay with his nephew, [[Ishikawa Kazumasa]]) as overall commander. The commanders of this unit consisted of many Matsudaira clansmen and other hereditary vassals which assigned on eastern side of the province, such as Shimada Heizo, [[Hiraiwa Chikayoshi]], [[Naitō Ienaga]], [[Sakai Tadatoshi]], Matsudaira Shinichi, and others.

===Tokugawa clan===
{{Main|Tokugawa clan}}
In 1567, Ieyasu started the family name "Tokugawa", changing his name to the well-known '''Tokugawa Ieyasu'''. As a member of the [[Matsudaira clan]], he claimed descent from the [[Seiwa Genji]] branch of the [[Minamoto clan]]. As there was no proof that the Matsudaira clan were descendants of [[Emperor Seiwa]],<ref>[[Timon Screech|Screech]], Timon (2006). ''Secret Memoirs of the Shoguns: Isaac Titsingh and Japan, 1779–1822''. London: [[RoutledgeCurzon]]. {{ISBN|0-7007-1720-X}}, p. 82.</ref> the Emperor initially did not approve the appointment, citing the lack of a precedent for the Serada clan of the Seiwa Genji clan to be appointed as ''Mikawa-no-kami'' (Lord of Mikawa).<ref name="taniguchi1994">{{Cite book|author=Taniguchi Kengo|title=流浪の戦国貴族 近衛前久 - 天下一統に翻弄された生涯 |trans-title=A Life at the Mercy of National Unification |series=中公新書|date=1994|pages=167–176}}</ref> 

Ieyasu then consulted with imperial noble Konoe Motohisa through the mediation of a Mikawa native and the abbot of the Kyo Seiganji Temple.<ref>{{cite web |title=徳川」への改姓と「家康」への改名」『徳川家康 将軍家蔵書からみるその生涯』|trans-title=Tokugawa Ieyasu: His Life as Seen in the Shogun Family Library |url=https://www.archives.go.jp/exhibition/digital/ieyasu/contents1_04/ |website=archives.go.jp |publisher=National archives of Japan |access-date=29 May 2024 |language=Ja}}</ref> Due to Motohisa's efforts, Yoshida Kaneyoshi discovered a genealogical document in the ''Manri-koji'' family that was precedent, saying, "Tokugawa (belongs) to Minamoto clan, as another offshoot of the Fujiwara clan," and a copy was transferred to him and used for the application.<ref name="taniguchi1994" /> Ieyasu then gained the permission of the Imperial Court and he was bestowed the [[courtesy title]] ''Mikawa-no-kami'' and the [[List of Japanese court ranks, positions and hereditary titles|court rank]] of {{Nihongo|Junior 5th Rank, Lower Grade|從五位下|ju go-i no ge}}. 

Though the [[Tokugawa clan]] could claim some modicum of freedom, they were very much subject to the requests of [[Oda Nobunaga]]. Ieyasu remained an ally of Nobunaga and his Mikawa soldiers were part of Nobunaga's army which captured Kyoto in 1568.

In 1568, at the same time, Ieyasu was eager to expand eastward to [[Tōtōmi Province]]. Ieyasu and [[Takeda Shingen]], the head of the [[Takeda clan]] in [[Kai Province]], made an alliance for the purpose of conquering all the Imagawa territory.<ref name="Sansom 1961"/>{{rp|279}} It is said{{Who|date=September 2024}} that the Tokugawa clan had made an agreement with the Takeda clan when dividing the territory that the eastern [[Suruga Province]] would become Takeda territory and the western Tōtōmi province would be Tokugawa territory, with the [[Ōi River]] as the border. 

On January 8, 1569, the Takeda vassal [[Akiyama Nobutomo]] invaded the Tōtōmi province from Shinano Province. The Takeda clan, through Oda Nobunaga, with whom they had a friendly relationship, asked Ieyasu, who was Nobunaga's ally, to reconsider cooperation with the Takeda, but Ieyasu rejected the idea, demonstrating a degree of independence from Nobunaga.

===Tōtōmi campaign===
In 1568, Ieyasu besieged the Horikawa Castle in Tōtōmi<ref name="OkazakiOkubotadachika59">{{cite book |title=新編岡崎市史: Sōshū hen Volume 20 of New edition of Okazaki City History |date=1983 |publisher=新編岡崎市史編さん委員会 |location=Okazaki City |page=59 |url=https://books.google.com/books?id=j3VMAQAAIAAJ |access-date=24 May 2024 |language=Ja}}</ref> and captured it in 1569. Ieyasu then ordered Ishikawa Hanzaburo to massacre the castle prisoners and residents, including women and children. It was recorded that around 700 people were beheaded on the banks of the Miyakoda River.<ref name="Horikawamassacre">* {{Cite book|last= Kamefumi Naito|date= 1972  |chapter= 姫街道の今昔 |editor=静岡新聞社 |title= ふるさと百話 |publisher= 静岡新聞社 |volume= 7 |pages= 155–159}}</ref> [[Ōkubo Tadachika]], who witnessed this massacre testified in his personal journal, ''Mikawa Monogatari'', that "... {{sic|both mens and womens can be cut into pieces}}...".<ref>{{citation |publisher=Hosoe town Board of Education |location=Horikawa castle Ruin |language=Ja |title=Mikawa Monogatari}}</ref>

Later the same year, Ieyasu's troops penetrated Tōtōmi Province. Meanwhile, Takeda Shingen's troops captured [[Suruga Province]], including the Imagawa capital of Sunpu. [[Imagawa Ujizane]] fled to [[Kakegawa Castle]], which led Ieyasu to [[Siege of Kakegawa|lay siege to Kakegawa]]. 

Ieyasu then negotiated with Ujizane, promising that if Ujizane surrendered himself and the remainder of Tōtōmi, Ieyasu would assist Ujizane in regaining Suruga. Ujizane had nothing left to lose, and Ieyasu immediately ended his alliance with [[Takeda clan|Takeda]], instead making a new alliance with Takeda's enemy to the north, [[Uesugi Kenshin]] of the [[Uesugi clan]]. Through these political manipulations, Ieyasu gained the support of the samurai of Tōtōmi Province.<ref name="Turnbull-I2"/> Furthermore, Ieyasu also placed the "''Iinoya's trio"'' (Iinoya-Sanninshu) of clans under the command of his trusted vassal, [[Ii Naomasa]].<ref name="shinshiro_iinoya">{{Cite web|url=https://www.city.shinshiro.lg.jp/kanko/hito/iinoya.html|title=井伊谷・井伊家と新城地域|publisher=新城市|access-date=2023-02-26}}</ref> The Iinoya trio were powerful clans that originated from the eastern side of Mikawa who greatly contributed to Ieyasu's expansion during his conquest of the former Imagawa territories in Tōtōmi Province.<ref>{{cite web |author1=Watanabe Daimon |title=徳川家康を遠江で手引きした「井伊谷三人衆」とは、いったい何者なのか |url=https://news.yahoo.co.jp/expert/articles/f996f723c6f02a99afad64fa625469e8c93f47c2 |website=yahoo.co.jp/expert/articles/ |publisher=渡邊大門 無断転載を禁じます。 © LY Corporation |access-date=3 June 2024 |language=Ja |date=2023}}</ref>

In 1570, Ieyasu established [[Hamamatsu]] as the capital of his territory, placing his son Matsudaira Nobuyasu in charge of [[Okazaki, Aichi|Okazaki]].<ref name="Arms">{{cite book|last1=Turnbull|first1=Stephen|title=Battles of the Samurai|date=1987|publisher=Arms and Armour Press|location=London|isbn=0853688265|pages=67–78}}</ref>{{efn|According to the Todai-ki, the castle was originally planned to be relocated to Mitsuke (the site of Kinosaki Castle is said to be), but was changed to Hamamatsu at the request of Oda Nobunaga. It is speculated that Nobunaga did not want the Oda and Tokugawa bases to be too far apart.<ref>{{harvtxt|Shiba|2021|p=190}} quoting {{cite book |last1=Koichi |first1=Abe |date=1997 |title=Shizuoka Prefecture History |chapter=Ieyasu's Conquest of Totomi and the Battle of Mikatagahara |series=General History |volume=2}}</ref>}}

=== Campaign against Asakura-Azai clan ===
{{Main|Siege of Kanegasaki (1570)|Battle of Anegawa}}

[[Asakura Yoshikage]], the head of the Asakura clan and regent of [[Ashikaga Yoshiaki]], refused to come to Kyōto. This caused Nobunaga to declare both of them rebels.<ref name=Sansom2>{{Cite book |last=Sansom |first=George |title=A History of Japan, 1334–1615 |publisher=Stanford University Press |year=1961 |isbn=0804705259 |page=281}}</ref> Several reports from ''Mikawa Monogatari'', ''Nobunaga Koki'', ''Tokugawa Jikki'', and a supplement from ''Ietada Nikki'' recorded that Ieyasu and his forces also participated in Oda's punitive campaign against Asakura, in which they captured Mount Tenzutsu castle. The Oda-Tokugawa forces managed to kill 1,370 enemies on April 25. 

They continued the next day by [[Siege of Kanegasaki (1570)|besieging Kanegasaki castle]].<ref>{{cite web |author1=小野 雅彦 |title=家康も撤退を知らされていなかった「金ヶ崎の退き口」 |url=https://www.rekishijin.com/27174 |website=Rekishijin |publisher=ABC ARC, inc. |access-date=10 June 2024 |language=Ja |date=2023 |quote=。織田・徳川連合軍は敵の首1370を討ち取るなどして城を攻め落とし（『家忠日記増補』）}}</ref> However the [[Azai clan]] sent reinforcements to relieve the siege so Nobunaga retreated without contacting Ieyasu. After dawn, Ieyasu withdrew from battle guided by Kinoshita Tokichi (later named [[Toyotomi Hideyoshi]]), an Oda vassal.<ref>{{cite web |author1=Koichiro Hamada (濵田 航一郎)  |title=金ヶ崎合戦、姉川の戦いで徳川家康は一体どうした⁉ |url=https://www.rekishijin.com/26842 |website=Rekishijin |publisher=ABC ARC, inc. |access-date=10 June 2024 |language=Ja |date=2023 |quote=。織田・徳川連合軍は敵の首1370を討ち取るなどして城を攻め落とし（『家忠日記増補』）}}</ref>

Later, in July 1570, the brother-in-law of Nobunaga, [[Azai Nagamasa]], who had broken his alliance with the Oda clan during the siege of Kanegasaki, and Asakura combined to fight the combined armies of Nobunaga and Ieyasu, who led 5,000 of his men to support Nobunaga at the battle.<ref name=Turnbull/>{{rp|62}} As the Oda clan engaged the Azai clan army on the right, Tokugawa's forces engaged the [[Asakura clan|Asakura clan's]] army on the left. At first, Asakura's army gained the advantage as they steadily pushed back the Tokugawa army. However, Honda Tadakatsu suddenly launched a lone, suicidal charge,<ref name="物語日本の歴史 16: 天下びとの時代Anegawa">{{cite book |author1=Kazuo Kasahara (笠原一男) |title=物語日本の歴史 16: 天下びとの時代 |date=1992 |publisher=木耳社 |isbn=4839375682 |page=193 |url=https://books.rakuten.co.jp/rb/514911/ |access-date=22 May 2024 |language=Ja}}</ref> while Sakakibara Yasumasa launched his force in a timely counterattack on Asakura's flank; together they managed to beat Asakura's forces.<ref name=Turnbull>{{Cite book |last=Turnbull |first=Stephen |title=Battles of the Samurai |publisher=Arms and Armour Press |date=1987 |isbn=0853688265 |pages=62–63}}</ref> Since Ieyasu's army was now free to move, they exploited the gap between Asakura and Azai's forces and sent Tadakatsu and Yasumasa to attack the flank of Azai's formation, which allowed the Oda-Tokugawa forces to win the battle.<ref name="The Samurai A Military History; Anegawa2013">{{harvtxt|Stephen Turnbull |2013 |p=140}}</ref>

=== Suruga campaign against Takeda clan ===
{{Main|Battle of Mikatagahara|Battle of Nagashino}}
In October 1571, Takeda Shingen broke the alliance with the Oda-Tokugawa forces and allied with the [[Late Hōjō clan|Odawara Hōjō clan]]. He decided to make a drive for Kyoto at the urging of the shōgun [[Ashikaga Yoshiaki]], starting by invading Tokugawa lands in Tōtōmi. Takeda Shingen's first objectives in his campaign against Ieyasu were [[Nishikawa Castle]], [[Yoshida Castle (Mikawa Province)|Yoshida Castle]] and [[Futamata Castle]]. 

In 1572, after [[Siege of Futamata|besieging Futamata]], Shingen would press on past Futamata towards the major Tokugawa home castle at [[Hamamatsu]]. Ieyasu asked for help from Nobunaga, who sent him some 3,000 troops. Early in 1573, the two armies met at the [[Battle of Mikatagahara]], north of Hamamatsu. The considerably larger Takeda army, under the expert direction of Shingen, overwhelmed Ieyasu's troops and caused heavy casualties. Despite his initial reluctance, Ieyasu was convinced by his generals to retreat.<ref name="Cassell">{{cite book|last1=Turnbull|first1=Stephen|title=The Samurai Sourcebook|date=2000|publisher=Cassell & Co|location=London|isbn=1854095234|pages=222–223}}</ref><ref name="Arms"/> 

The battle was a major defeat, but in the interest of maintaining the appearance of a dignified withdrawal, Ieyasu brazenly ordered the men in his castle to light torches, sound drums, and leave the gates open, to properly receive the returning warriors. To the surprise and relief of the Tokugawa army, this spectacle made the Takeda generals suspicious that they were being led into a trap, so they did not besiege the castle and instead made camp for the night.<ref name="Cassell"/> This error allowed a band of Tokugawa soldiers to raid the camp in the ensuing hours, further upsetting the already disoriented Takeda army, and ultimately resulting in Shingen's decision to call off the offensive altogether. Takeda Shingen would not get another chance to advance on Hamamatsu, much less Kyoto, since he died from unknown causes shortly after the [[siege of Noda Castle]] later that same year.<ref name="sam"/>{{rp|153–156}}

In 1574, Shingen was succeeded by his son [[Takeda Katsuyori]] and the conflict continued as the Tokugawa forces under Honda Tadakatsu and Sakakibara Yasumasa seized many of the Takeda clan's castles,<ref name="Nagashino1573;TadakatsuYasumasa">{{cite book |author1=Murata Shuzo |author2=Hattori Hideo |title=都道府県別日本の中世城館調査報告書集成 |trans-title=Collection of Japanese medieval castle survey reports by prefecture |date=2000 |publisher=東洋書林 |isbn=4887214324 |page=418 |url=https://ci.nii.ac.jp/ncid/BA49003233 |access-date=22 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Shin Jinbutsu Ōraisha |title=歴史読本, Volume 52, Issue 3 |trans-title=History Reader, Volume 52, Masalah |date=2007 |publisher=新人物往来社 |page=92 |url=https://books.google.com/books?id=v1ozAQAAIAAJ |access-date=22 May 2024 |language=Ja}}</ref> including Komyo Castle.<ref name="Research establishment">{{cite book |author1=煎本 増夫 |title=幕藩体制成立史の研究 |trans-title=Research on the history of the establishment of the Shogunate system |date=1979 |publisher=雄山閣出版 |page=94 |url=https://bookmeter.com/books/1502589 |access-date=15 May 2024 |language=Ja}}</ref><ref name="Research of Fudai">{{cite book |author1=Naito Family Documents Study Group |title=譜代藩の研究 : 譜代内藤藩の藩政と藩領 |trans-title=Research on the Fudai domain: the government and domain of the Fudai Naito domain |date=1972 |publisher=Meiji University |page=44 |url=https://ndlsearch.ndl.go.jp/books/R100000002-I000001212352 |access-date=15 May 2024 |language=Ja}}</ref> At some point, Ieyasu tried to capture Inui Castle in Tōtōmi Province, but strong resistance from its garrison commander, Amano Kagehira, forced Ieyasu to abort the siege. During their retreat, Kagehira launched a counterattack to pursue Ieyasu, but this was repelled by [[Mizuno Tadashige]] and Torii Mototada, who led the rearguard.<ref>{{cite book|last=Hotta|first=Masaatsu|date=1923| title =寛政重脩諸家譜. 第2輯|publisher =國民圖書|language=Ja |page=823}}</ref>

In April 1575, Ōga Yashirō, a deputy governor of over 20 villages in Oku district of Mikawa under Matsudaira Nobuyasu,<ref name="Hamada Koichiro; Yashiro incident" /> was arrested by [[Ōkubo Tadayo]] and paraded around [[Hamamatsu Castle]]. According to the investigation, Yashirō was implicated in allegations of colluding with Takeda Katsuyori to betray Ieyasu and invade the Tokugawa clan's territory. According to a letter, Yashiro had teamed up with Takeda Katsuyori of Kai to seize Okazaki Castle. However, one of Yashiro's colleagues, Yamada Hachizō, betrayed Yashiro and passed this information to Nobuyasu. Meanwhile, Ieyasu himself also learned about Yashirō's further crimes of corruption in governance through a vassal's report.<ref name="Hamada Koichiro; Yashiro incident">{{cite web |author1=Hamada Kōichirō (濱田浩一郎) |title=「どうする家康」『徳川実紀』が描く大岡弥四郎事件 岡崎クーデターはなぜ失敗したのか？ |trans-title="What would you do, Ieyasu?" The Ōga Yashirō Incident as described in the Tokugawa Jikki: Why did the Okazaki Coup fail? |url=https://sengoku-his.com/694 |website=Rekishijin |publisher=Abc Arc, inc. |access-date=9 June 2024 |language=Ja |date=2023}}</ref> Yashirō was then executed by being mutilated alive with a saw, while Tadayo crucified his wife and children.<ref name="OgaYashiro; Watanabe Daimon1">{{cite web |author1=Watanabe Daimon |title=残酷すぎて放映できなかった、大岡弥四郎のあまりにむごたらしい最期 |url=https://news.yahoo.co.jp/expert/articles/df6862aa19e2d4a45354f63f4cd6a4c22a740cfc |website=yahoo.co.jp/expert/articles/ |publisher=渡邊大門 無断転載を禁じます。 © LY Corporation |access-date=4 June 2024 |language=Ja}}</ref> 

In June, during Takeda Katsuyori's raid on Mikawa Province when he [[Siege of Yoshida Castle|attacked Yoshida Castle]] and [[Battle of Nagashino|besieged Nagashino Castle]], Ieyasu appealed to Nobunaga for help; Nobunaga came personally with 30,000 men. The Oda-Tokugawa forces 38,000 strong won a great victory and successfully defended Nagashino Castle. Though the Takeda forces had been destroyed, Katsuyori survived the battle and retreated to [[Kai Province]].<ref>{{Cite book|last=Turnbull|first=Stephen|url=https://books.google.com/books?id=savvCwAAQBAJ&q=temple+on+the+road+back+towards+kai|title=Nagashino 1575: Slaughter at the barricades|date=2012|publisher=Bloomsbury Publishing|isbn=978-1-78200-229-1|pages=79|language=en}}</ref> For the next seven years, Ieyasu and Katsuyori fought a series of small battles through which Ieyasu's troops managed to wrest control of [[Suruga Province]] from the Takeda clan.

=== Nobuyasu Incident ===
[[File:築山殿.jpg|thumb|Tsukiyama-Dono, wife of Ieyasu who was executed due to an accusation of treason]]
In 1579, [[Lady Tsukiyama]], Ieyasu's wife, and his heir Matsudaira Nobuyasu were accused by Nobunaga of conspiring with Takeda Katsuyori to assassinate Nobunaga, whose daughter Tokuhime was married to Nobuyasu. Ieyasu ordered his wife to be executed and forced his son to commit ''[[seppuku]]'' because of these accusations.<ref name="Ōoku The Secret World of the Shogun's Women; Tadatsugu Sakai">{{cite book |author1=Cecilia Segawa Seigle |author2=Linda H. Chance |title=Ōoku The Secret World of the Shogun's Women |date=2014 |publisher=Cambria Press |isbn=9781604978728 |url=https://books.google.com/books?id=nbF5CgAAQBAJ |access-date=6 May 2024 |language=En |quote=Notes 23= Tsukiyama-Dono pp. 85–86; Kinseishi p. 30}}</ref><ref name="Shogun The Life of Tokugawa Ieyasu; Lady Tsukiyama">{{cite book |author1=Arthur Lindsay Sadler |author1-link=Arthur Lindsay Sadler |title=Shogun The Life of Tokugawa Ieyasu |date=2004 |publisher=Tuttle Publishing |isbn=9781462916542 |page=53 |url=https://books.google.com/books?id=5HhzBgAAQBAJ |access-date=6 May 2024 |language=En}}</ref>

There are various theories regarding this incident. According to the "Mikawa Monogatari'", which was written by [[Ōkubo Tadachika]], Tokuhime, wife of Nobuyasu, who was not on good terms with her mother-in-law Tsukiyama-dono, wrote in a letter to her father, Nobunaga, that her mother-in-law and her husband were secretly conspiring with Takeda Katsuyori.<ref name="Tetsuo Owada 2002 24">{{harvtxt|Tetsuo Owada|2002|p=24}}</ref>

However, this hypothesis has been considered implausible by various historians in the modern era. According to Katsuhiro Taniguchi, the more plausible theory was that there was friction within the house of Tokugawa clan between two factions with conflicting ideals, as proposed by the writer Tenkyu Goro. One faction, dubbed the "Hamamatsu Castle Faction," was active on the front lines and had many opportunities to advance their careers. The other faction was the "Okazaki Castle Faction," which consisted of Tokugawa vassals responsible for logistical support due to past injuries and other factors that caused them to play a lesser role in the politics of Tokugawa clan. According to this theory, the conflict between these two factions eventually led to a conflict between Ieyasu, representing the Hamamatsu faction, and his son Nobuyasu, representing the Okazaki faction, finally ending with Nobuyasu's death in prison. Nobuyasu planned to exile Ieyasu with the help of the Okazaki Castle faction. Before and after his son's execution, Ieyasu punished or executed many of those who worked at Okazaki Castle, although some escaped. Taniguchi theorized that Tsukiyama also participated in the coup d'état that was going on in Okazaki Castle.{{sfn|Taniguchi|2007|pp=209–211}} Furthermore, Sakai Tadatsugu, the most prominent general of Ieyasu, also may have played a role in confirming Oda Nobunaga's suspicion of the alleged betrayal against the Oda clan being planned by Nobuyasu Tsukiyama. Ieyasu may have concluded that if a high-ranking [[fudai daimyō]] such as Tadatsugu had confirmed the accusations against Lady Tsukiyama, then they must be true.<ref name="Ōoku The Secret World of the Shogun's Women; Tadatsugu Sakai"/>

Another theory has said that Tadatsugu was actually conspiring with the Ieyasu's mother, [[Odai no Kata]], to get rid of Lady Tsukiyama.<ref name="Ōoku The Secret World of the Shogun's Women; Tadatsugu Sakai"/> [[Arthur Lindsay Sadler]] theorized this was a deliberate act of spite from Tadatsugu due to many senior Tokugawa clan generals' dislike of Nobuyasu.<ref name="Shogun The Life of Tokugawa Ieyasu; Lady Tsukiyama"/>

In the same year, Ieyasu named his third son, [[Tokugawa Hidetada]], as his heir, since his second son had been adopted by [[Toyotomi Hideyoshi]], who would later become a powerful daimyo.{{Citation needed|date=March 2020}}

=== Takeda clan annihilation ===
{{Main|Siege of Takatenjin (1581)|Battle of Tenmokuzan}}

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|text=Positions of the Takatenjin Castle and the six forts<br>1.''Ogasayama Fort,'' 2.''Nogasaka Fort'', 3.''Higamine Fort,''4. ''Shishigahana Fort'', 5.''Nakamura Fort'', 6.'''Mitsuiyama Fort'', 7.''Takatenjin Castle''}}
In 1580, Ieyasu built five fortresses in order to isolate Takatenjin Castle from external supplies and reinforcements.<ref name="kakegawa20171006">{{cite web |title=高天神城と六砦 |trans-title=Takatenjin Castle and Six Fortresses |url=https://www.city.kakegawa.shizuoka.jp/gyosei/docs/8327.html |website=city.kakegawa.shizuoka.jp |publisher=Kakegawa City |access-date=28 May 2024 |location=Kakegawa City, [[Shizuoka (city)|Shizuoka]] |language=Ja |date=2019}}</ref><ref name="mizuno20150920">{{cite web |author1=Mizuno Shigeru |title=大坂砦 |trans-title=Osaka Fort |url=https://www.sankei.com/article/20150920-IA4VDCQT2NJMTK727CAET3RULA/ |website=sankei.com |publisher=The Sankei Shimbun |access-date=28 May 2024 |language=Ja |date=2015}}</ref><ref name="mizuno20150927">{{cite web |author1=Mizuno Shigeru |title=小笠山砦 |trans-title=Ogasayama Fort |url=https://www.sankei.com/article/20150927-IXWVQDD66VJKFKVXKYU53TIRAU/ |website=sankei.com |publisher=The Sankei Shimbun |access-date=28 May 2024 |language=Ja |date=2015}}</ref> In addition to those five new fortresses, Ieyasu renovated an old castle named Ogasayama fort, which had originally been built by Ieyasu far before the conquest of Tōtōmi Province against Imagawa clan to capture [[Kakegawa Castle]].<ref name="mizuno20150927" /> With the six fortresses, which were referred to as the "six fortresses of Takatenjin", completed, Ieyasu assigned Ishikawa Yasumichi to the Ogasayama fort, [[Honda Yasushige (1554-1611)|Honda Yasushige]] to the Nogasaka fort, Osuga Yasutaka to the Higamine,<ref name="kakegawa20171006" /> Shishigahana,<ref>{{cite web |title=獅子ヶ鼻砦跡 |trans-title=Shishigahana Fort Ruins |url=https://www.city.kikugawa.shizuoka.jp/shoukoukankou/shishigahana.html |website=city.kikugawa.shizuoka.jp |publisher=Kikugawa City |access-date=28 May 2024 |date=2022}}</ref> and Nakamura forts,<ref name="kakegawa20171006" /> while Sakai Ietada was appointed to garrison the Mitsuiyama fort.<ref name="mizuno20150920" /><ref name="kakegawa20171006" /> 

The Takatenjin castle which was defended by [[Okabe Motonobu]], immediately suffered from a period of starvation as the siege by Oda-Tokugawa forces intensified.<ref name="kakegawa20171006" /> In response, Motonobu tried to negotiate a truce with Ieyasu by offering Takisakai and Koyama Castle in exchange for Takatenjin castle being spared from the siege.<ref>{{Cite book |author = Marushima Kazuhiro |title = 武田勝頼 |series = 中世から近世へ |publisher = 平凡社|date = 2017}}</ref> After Ieyasu consulted with Nobunaga on this matter, he refused Motonobu's plea. Nobunaga stated his reason in a letter saying that if Takeda Katsuyori sent his forces to assist Motonobu, then there would be an opportunity to bait the Takeda army into coming outside the castle and annihilate them on the field. Meanwhile, Nobunaga also stated that if Katsuyori neglected helping Motonobu at all, it would damage the Takeda clan's credibility because they could not save their own vassals.<ref>{{cite book |author1=Okuno Takahiro |title=増訂 織田信長文書の研究 上巻  |trans-title=Expanded study of Oda Nobunaga's documents, Volume 1 |date=2007 |publisher=吉川弘文館 |isbn=9784642009072 |url=https://www.yoshikawa-k.co.jp/book/b77694.html |access-date=28 May 2024 |language=Ja}}</ref>

In 1581, Ieyasu forces managed to subdue Tanaka castle,<ref name="Tadakatsu;Kajiwara Ai">{{cite book |title=ð̇Æ̇̌Þ̄ʹđ̇: Rekicho yoki |date=1998 |page=九日圍:田中城、井伊直政歲十八 |publisher=Æ̇̌Þ̄ʹđ̇ʺ̇·ð̇ỡ |url=https://books.google.com/books?id=7JwJAQAAMAAJ |access-date=19 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Kajiwara Ai |author2=Matsudaira Yoriyasu |date=2003 |publisher=神道大系編纂会 |title=田中城、井伊直政歲十八 |url=https://books.google.com/books?id=R0EyAQAAIAAJ |access-date=19 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Kimura Takaatsu |title=武徳編年集成 |date=1976 |publisher=名著出版 |page=229 |url=https://ndlsearch.ndl.go.jp/books/R100000002-I000001222955 |access-date=19 May 2024 |language=Ja}}</ref><ref>{{cite book |author1=Motonao Narushima |author2=Tadachika Kuwata |author3=Takehisa Udagawa |title=改正三河後風土記 Volume 2 |trans-title=Revised Mikawa Go Fudoki Volume 2 |date=1976 |publisher=秋田書店 |page=110 |language=Ja}}</ref> and [[Siege of Takatenjin (1581)|recapture Takatenjin castle]], where Okabe Motonobu was killed during the fight.<ref name=kotobank>{{cite web|url=https://kotobank.jp/word/%E5%B2%A1%E9%83%A8%E9%95%B7%E6%95%99-1062236|title=デジタル版 日本人名大辞典+Plus「岡部長教」の解説|publisher=kotobank|access-date=20 October 2021}}</ref>

The end of the war with Takeda Katsuyori came in 1582, when a combined Oda-Tokugawa force attacked and conquered [[Kai Province]], where Katsuyori was cornered and defeated at the [[Battle of Tenmokuzan]], and then committed ''seppuku''.{{sfn|Turnbull |1998 |p=231}} With the Takeda clan practically annihilated as political entity, Nobunaga gave Ieyasu the right to govern [[Suruga Province]] in recognition of his service in the fight against the Takeda clan.<ref name="Susumu Shimazaki Suruga">{{cite web |author1=Susumu Shimazaki (島崎晋)|author2=Rekishijin Editorial Department |title=徳川家康が武田氏を倒したとき、織田信長が褒美にくれた領地は広かった！ |trans-title=When Tokugawa Ieyasu defeated the Takeda clan, the territory that Oda Nobunaga gave him as a reward was vast! |url=https://www.rekishijin.com/26846 |website=Rekishijin |publisher=ABC ARC, Inc. |access-date=24 June 2024 |language=Ja |date=2023 |quote=From 'The Truth About Tokugawa Ieyasu' in the February 2023 issue of Rekishijin article}}</ref>