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.

[2] 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.

[8] 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.

[10][11] 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.

[14] 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.

[23] Mexican authorities were initially content to govern the remote province through salutary neglect, "permitting slavery under the legal fiction of 'permanent indentured servitude', similar to Mexico's peonage system.

[18] In the years following independence, the migration of white settlers and importation of black slave labor into the vast republic was deterred by Texas's unresolved international status and the threat of renewed warfare with Mexico.

[38] But, despite his strong support for Texas independence from Mexico,[39] then-President Andrew Jackson delayed recognizing the new republic until the last day of his presidency to avoid raising the issue during the 1836 general election.

Tyler, isolated and outside the two-party mainstream, turned to foreign affairs to salvage his presidency, aligning himself with a southern states' rights faction that shared his fervent slavery expansionist views.

[48] In his first address to Congress in special session on June 1, 1841, Tyler set the stage for Texas annexation by announcing his intention to pursue an expansionist agenda so as to preserve the balance between state and national authority and to protect American institutions, including slavery, so as to avoid sectional conflict.

[57] In the spring of 1843, the Tyler administration had sent executive agent Duff Green to Europe to gather intelligence and arrange territorial treaty talks with Great Britain regarding Oregon; he also worked with American minister to France, Lewis Cass, to thwart efforts by major European powers to suppress the maritime slave trade.

His investigations, including personal interviews with Lord Aberdeen, concluded that British interest in abolitionist intrigues was weak, contradicting Secretary of State Upshur's conviction that Great Britain was manipulating Texas.

[60] Though unsubstantiated, Green's unofficial intelligence so alarmed Tyler that he requested verification from the US minister to Mexico, Waddy Thompson Jr.[61] John C. Calhoun of South Carolina, a pro-slavery Democrat,[62] counseled Secretary Upshur that British designs on American slavery were real and required immediate action to preempt a takeover of Texas by the United Kingdom.

[72] Still, US Secretary of State Upshur vigorously exhorted Texas diplomats to begin annexation talks, finally dispatching an appeal to President Sam Houston in January 1844.

Mexican minister to the U.S. Juan Almonte confronted Upshur with these reports, warning him that if Congress sanctioned a treaty of annexation, Mexico would break diplomatic ties and immediately declare war.

[81] The Princeton disaster proved a major setback for Texas annexation, in that Tyler expected Secretary Upshur to elicit critical support from Whig and Democratic Senators during the upcoming treaty ratification process.

The choice of Calhoun, a highly regarded but controversial American statesman,[83] risked introducing a politically polarizing element into the Texas debates, but Tyler prized him as a strong advocate of annexation.

[86] In it, Walker argued that Texas could be acquired by Congress in a number of ways – all constitutional – and that the moral authority to do so was based on the precepts for territorial expansion established by Jefferson and Madison, and promulgated as doctrine by Monroe in 1823.

He envisioned Texas as a corridor through which both free and enslaved African-Americans could be "diffused" southward in a gradual exodus that would ultimately supply labor to the Central American tropics, and in time, empty the United States of its slave population.

[88] This "safety-valve" theory "appealed to the racial fears of northern whites" who dreaded the prospect of absorbing emancipated slaves into their communities if the institution of slavery collapsed in the South.

[89] This scheme for racial cleansing was consistent, on a pragmatic level, with proposals for overseas colonization of blacks, which were pursued by a number of American presidents, from Jefferson to Lincoln.

[90] Walker bolstered his position by raising national security concerns, warning that in the event annexation failed, Great Britain would maneuver the Republic of Texas into emancipating its slaves, forecasting a dangerous destabilizing influence on southwestern slaveholding states.

[98] Any allusion to slavery was omitted from the document so as not to antagonize anti-slavery sentiments during Senate debates, but it provided for the "preservation of all [Texas] property as secured in our domestic institutions.

"[99] Upon the signing of the treaty, Tyler complied with the Texans' demand for military and naval protection, deploying troops to Fort Jesup in Louisiana and a fleet of warships to the Gulf of Mexico.

[102] Secretary of the Treasury John C. Spencer was alarmed at the constitutional implications of Tyler's application of military force without congressional approval, a violation of the separation of powers.

[160] President-elect Polk had expressed his ardent wish that Texas annexation should be accomplished before he entered Washington in advance of his inauguration on March 4, 1845, the same day Congress would end its session.

[165] Benton and his allies were assured that Polk would act to establish the eastern portion of Texas as a slave state; the western section was to remain unorganized territory, not committed to slavery.

[169] But, during his last full day in office, President Tyler, with the urging of his Secretary of State Calhoun,[170] decided to act decisively to improve the odds for the immediate annexation of Texas.

Taylor moved into Texas, ignoring Mexican demands to withdraw, and marched as far south as the Rio Grande, where he began to build a fort near the river's mouth on the Gulf of Mexico.

Following a United States victory and the signing of the Treaty of Guadalupe Hidalgo, Mexico ceded its claims to Texas and the Rio Grande border was accepted by both nations.