Editing Sam Houston (section)

===Mexican–American War and aftermath (1846–1853)===
{{see also|Presidency of James K. Polk|Presidency of Millard Fillmore}}
[[File:United States 1849-1850.png|thumb|upright=1.5|The United States in 1849, with the full extent of Texas's land claims shown]]

In February 1846, the Texas legislature elected Houston and Thomas Jefferson Rusk as Texas's two inaugural U.S. senators. Houston chose to align with the Democratic Party, which contained many of his old political allies, including President Polk.{{sfn|Haley|2002|pp=294–295}} As a former president of Texas, Houston is the only former foreign head of state to have served in the U.S. Congress.{{citation needed|date=November 2018}} He was the first person to serve as the governor of a state and then be elected to the U.S. Senate by another state. In 2018, [[Mitt Romney]] became the second.<ref name=":0">{{cite web|url=http://editions.lib.umn.edu/smartpolitics/2017/09/14/mitt-romney-prepares-for-unusual-us-senate-bid/|title=Mitt Romney Prepares for Unusual US Senate Bid {{!}} Smart Politics|website=editions.lib.umn.edu|date=September 14, 2017|language=en-CA|access-date=April 1, 2018}}</ref>
[[William Wyatt Bibb|William W. Bibb]] accomplished the same feat in reverse order. 

Breaking with the Senate tradition that held that freshman senators were not to address the Senate, Houston strongly advocated in early 1846 for the annexation of [[Oregon Country]]. In the [[Oregon Treaty]], reached later in 1846, Britain and the United States agreed to split Oregon Country.{{sfn|Haley|2002|p=297}} Meanwhile, Polk ordered General [[Zachary Taylor]] to lead a U.S. army to the [[Rio Grande]], which had been set as the Texas-Mexico border under the Treaty of Velasco; Mexico claimed the Nueces River constituted the true border. After a skirmish between Taylor's unit and the Mexican army, the [[Mexican–American War]] broke out in April 1846. Houston initially supported Polk's prosecution of the war, but differences between the two men emerged in 1847.{{sfn|Haley|2002|pp=297–298}} After two years of fighting, the United States defeated Mexico and, through the [[Treaty of Guadalupe Hidalgo]], acquired the [[Mexican Cession]]. Mexico also agreed to recognize the Rio Grande as the border between Mexico and Texas.<ref>{{Cite web|title=Treaty of Guadalupe Hidalgo|url=https://www.history.com/topics/mexican-american-war/treaty-of-guadalupe-hidalgo|access-date=January 13, 2021|website=HISTORY|language=en}}</ref>

After the war, disputes over the extension of slavery into the [[United States territory|territories]] raised sectional tensions. Unlike most of his Southern colleagues, Houston voted for the [[Oregon Bill of 1848]], which organized [[Oregon Territory]] as a free territory. Defending his vote to create a territory that excluded slavery, Houston stated "I would be the last man to wish to do anything injurious to the South, but I do not think that on all occasions we are justified in agitating [slavery]."{{sfn|Haley|2002|p=301}} He criticized both Northern [[abolitionism in the United States|abolitionists]] and Democratic followers of Calhoun as extremists who sought to undermine the union.{{sfn|Haley|2002|pp=302–304}} He supported the [[Compromise of 1850]], a sectional compromise on slavery on the territories. Under the compromise, California was admitted as a free state, the slave trade was prohibited in the District of Columbia, a more stringent [[Fugitive Slave Act of 1850|fugitive slave law]] was passed, and [[Utah Territory]] and [[New Mexico Territory]] were established. Texas gave up some of its claims on New Mexico, but it retained El Paso, Texas, and the United States assumed Texas's large public debt.{{sfn|Haley|2002|pp=304–308}} Houston sought the Democratic nomination in the [[1852 United States presidential election|1852 presidential election]], but he was unable to consolidate support outside of his home state. The [[1852 Democratic National Convention]] ultimately nominated [[Franklin Pierce]], a compromise nominee, who went on to win the election.{{sfn|Haley|2002|pp=311–313}}