Texas annexation

The Republic of Texas was the only state to enter by treaty into the United States and admitted to the Union as the 28th state on December 29, 1845.

The Republic of Texas declared independence from the Republic of Mexico on March 2, 1836. It applied for annexation to the United States the same year, but was rejected by the United States Secretary of State, John Forsyth, under President Andrew Jackson. At that time, the majority of the Texian population favored the annexation of the Republic by the United States. The leadership of both major U.S. political parties (the Democrats and the Whigs) opposed the introduction of Texas – a vast slave-holding region – into the volatile political climate of the pro- and anti-slavery sectional controversies in Congress. Moreover, they wished to avoid a war with Mexico, whose government had outlawed slavery and refused to acknowledge the sovereignty of its rebellious northern province. With Texas's economic fortunes declining by the early 1840s, the President of the Texas Republic, Sam Houston, arranged talks with Mexico to explore the possibility of securing official recognition of independence, with the United Kingdom mediating.

In 1843, U.S. President John Tyler, then unaligned with any political party, decided independently to pursue the annexation of Texas in a bid to gain a base of support for another four years in office. His official motivation was to outmaneuver suspected diplomatic efforts by the British government for the emancipation of slaves in Texas, which would undermine slavery in the United States. Through secret negotiations with the Houston administration, Tyler secured a treaty of annexation in April 1844. When the documents were submitted to the U.S. Senate for ratification, the details of the terms of annexation became public and the question of acquiring Texas took center stage in the presidential election of 1844. Pro-Texas-annexation southern Democratic delegates denied their anti-annexation leader Martin Van Buren the nomination at their party's convention in May 1844. In alliance with pro-expansion northern Democratic colleagues, they secured the nomination of James K. Polk, who ran on a pro-Texas Manifest destiny platform.

In June 1844, the Senate, with its Whig majority, soundly rejected the Tyler–Texas treaty. Later that year, the pro-annexation Democrat Polk narrowly defeated anti-annexation Whig Henry Clay in the 1844 presidential election. In December 1844, lame-duck President Tyler called on Congress to pass his treaty by simple majorities in each house. The Democratic-dominated House of Representatives complied with his request by passing an amended bill expanding on the pro-slavery provisions of the Tyler treaty. The Senate narrowly passed a compromise version of the House bill, designed to provide President-elect Polk the options of immediate annexation of Texas or new talks to revise the annexation terms of the House-amended bill.

On March 1, 1845, President Tyler signed the annexation bill, and on March 3 (his last full day in office), he forwarded the House version to Texas, offering immediate annexation. When Polk took office at noon the following day, he encouraged Texas to accept Tyler’s offer. Texas ratified the agreement with popular approval from Texians. The bill was signed by President Polk on December 29, 1845, accepting Texas as the 28th state of the Union. Texas formally joined the union on February 19, 1846, prompting the Mexican–American War in April of that year.

First mapped by Spain in 1519, for over 300 years Texas was part of the vast Spanish Empire seized by the Spanish conquistadores from its indigenous people. The US-Spain border along the northern frontier of Texas took shape in the 1817–1819 negotiations between Secretary of State John Quincy Adams and the Spanish ambassador to the United States, Luis de Onís. The boundaries of Texas were determined within the larger geostrategic struggle to demark the limits of the United States' extensive western lands and of Spain's vast possessions in North America. The Florida Purchase Treaty of February 22, 1819 emerged as a compromise that excluded Spain from the lower Columbia River drainage basin, but established southern boundaries at the Sabine and Red Rivers, "legally extinguish[ing]" any American claims to Texas. Nonetheless, Texas remained an object of fervent interest to American expansionists, among them Thomas Jefferson, who anticipated the eventual acquisition of its fertile lands.

The Missouri crisis of 1819–1821 sharpened commitments to expansionism among the country's slaveholding interests, when the so-called Thomas proviso established the 36°30' parallel, imposing free-soil and slave-soil futures in the Louisiana Purchase lands. While a majority of southern congressmen acquiesced to the exclusion of slavery from the bulk of the Louisiana Purchase, a significant minority objected. Virginian editor Thomas Ritchie of the Richmond Enquirer predicted that with the proviso restrictions, the South would ultimately require Texas: "If we are cooped up on the north, we must have elbow room to the west." Representative John Floyd of Virginia in 1824 accused Secretary of State Adams of conceding Texas to Spain in 1819 in the interests of Northern anti-slavery advocates, and so depriving the South of additional slave states. Then-Representative John Tyler of Virginia invoked the Jeffersonian precepts of territorial and commercial growth as a national goal to counter the rise of sectional differences over slavery. His "diffusion" theory declared that with Missouri open to slavery, the new state would encourage the transfer of underutilized slaves westward, emptying the eastern states of bondsmen and making emancipation feasible in the old South. This doctrine would be revived during the Texas annexation controversy.

When Mexico won its independence from Spain in 1821, the United States did not contest the new republic's claims to Texas, and both presidents John Quincy Adams (1825–1829) and Andrew Jackson (1829–1837) persistently sought, through official and unofficial channels, to procure all or portions of provincial Texas from the Mexican government, without success.