Mexico and Texas[edit]
Manifest Destiny played an important role in the expansion of Texas and American relationship with Mexico. In 1836, the Republic of Texas declared independence from Mexico and, after the Texas Revolution, sought to join the United States as a new state. This was an idealized process of expansion which had been advocated from Jefferson to O'Sullivan: newly democratic and independent states would request entry into the United States, rather than the United States extending its government over people who did not want it. The annexation of Texas was controversial as it would add another slave state to the Union. Presidents Andrew Jackson and Martin Van Buren declined Texas's offer to join the United States in part because the slavery issue threatened to divide the Democratic Party.
Before the election of 1844, Whig candidate Henry Clay and the presumed Democratic candidate, former President Van Buren, both declared themselves opposed to the annexation of Texas, each hoping to keep the troublesome topic from becoming a campaign issue. This unexpectedly led to Van Buren being dropped by the Democrats in favor of Polk, who favored annexation. Polk tied the Texas annexation question with the Oregon dispute, thus providing a sort of regional compromise on expansion. (Expansionists in the North were more inclined to promote the occupation of Oregon, while Southern expansionists focused primarily on the annexation of Texas.) Although elected by a very slim margin, Polk proceeded as if his victory had been a mandate for expansion.