Editing Treaty of Guadalupe Hidalgo (section)

==Background to the war==
[[File:Negociación de la frontera México-EUA.svg|thumb|right|300px|Map of negotiation of the border between Mexico and the United States (1845-1848) as part of the American intervention in México.]]

Mexico had claimed the area in question since winning its independence from the [[Spanish Empire]] in 1821 following the [[Mexican War of Independence]]. The Spanish had conquered part of the area from the American Indian tribes over the preceding three centuries. Still, powerful and independent indigenous nations remained within that northern region of Mexico.{{citation needed|date=March 2023}}

Most of that land was too dry and too mountainous to support a large population. About 80,000 Mexicans inhabited California, New Mexico, Arizona, and Texas during the period 1845 to 1850, with far fewer in [[Nevada]], southern and western Colorado, and Utah.<ref>{{cite journal | last1 = Nostrand | first1 = Richard L. | year = 1975 | title = Mexican Americans Circa 1850 | journal = Annals of the Association of American Geographers | volume = 65 | issue = 3| pages = 378–390 | doi = 10.1111/j.1467-8306.1975.tb01046.x }}</ref> On 1 March 1845, U.S. President [[John Tyler]] signed legislation to authorize the United States to [[Texas annexation|annex the Republic of Texas]], effective on 29 December 1845. The Mexican government, which had never recognized the [[Republic of Texas]] as an independent country, had warned that annexation would be viewed as an [[casus belli|act of war]]. Both the United Kingdom and France recognized the Republic of Texas's independence and repeatedly tried to dissuade Mexico from declaring war against its northern neighbor. British efforts to mediate the quandary proved fruitless, in part because other political disputes (particularly the [[Oregon boundary dispute]]) arose between Great Britain (as the claimant of modern Canada) and the United States.

On 10 November 1845, before the outbreak of hostilities, President [[James K. Polk]] sent his envoy, [[John Slidell]], to Mexico. Slidell had instructions to offer Mexico around $5 million for the territory of [[Santa Fe de Nuevo México|Nuevo México]] and up to $40 million for [[Alta California]].<ref>Mills, B. 2003. ''U.S.-Mexican War.'' Facts On File, p. 23. {{ISBN|0-8160-4932-7}}</ref> The Mexican government dismissed Slidell, refusing to even meet with him.<ref name="Polk Third Annual Message">{{cite web|url=http://www.presidency.ucsb.edu/ws/index.php?pid=29488&st=&st1=|title=James K. Polk's Third Annual Message, 7 December 1847|publisher=presidency.ucsb.edu|access-date=27 June 2007}}</ref> Earlier in that year, Mexico had broken off [[diplomatic relations]] with the United States, based partly on its interpretation of the [[Adams–Onís Treaty]] of 1819, under which newly independent Mexico claimed it had inherited rights. In that agreement, the United States had "renounced forever" all claims to Spanish territory.<ref>[http://www.yale.edu/lawweb/avalon/diplomacy/spain/sp1819.htm#art3 Adams-Onis Treaty, Article III.] {{webarchive|url=https://web.archive.org/web/20060719062738/http://www.yale.edu/lawweb/avalon/diplomacy/spain/sp1819.htm |date=19 July 2006 }} From: yale.edu. Retrieved 6 November 2007.</ref><ref>"The United States hereby cede to His Catholic Majesty, and renounce forever, all their rights, claims, and pretensions to the Territories lying West and South of the above described Line [...]. http://www.tamu.edu/faculty/ccbn/dewitt/adamonis.htm</ref>

Neither side took any further action to avoid a war. Meanwhile, Polk settled a major territorial dispute with Britain via the [[Oregon Treaty]], which was signed on 15 June 1846. By avoiding any chance of conflict with Great Britain, the United States was given a free hand regarding Mexico. After the [[Thornton Affair]] of 25–26 April, when Mexican forces attacked an American unit in the disputed area, with the result that 11 Americans were killed, five wounded, and 49 captured, Congress passed a declaration of war, which Polk signed on 13 May 1846. The Mexican Congress responded with its own war declaration on 23 April 1846.{{Citation needed|reason=Illogical as a purported Mexican response, as date of Mexican declaration predates Thornton Affair|date=September 2022}}{{sfn|Davenport|2005|p=39}}