Prior to 1824 Texas was a Spanish possession and thereafter part of independent Mexico. Americans began to settle in Texas beginning in 1821 when Spanish authorities allowed American to acquire land in the sparsely settled region. Although prospective settlers were required to be Catholic, conduct their affairs in Spanish, and swear allegiance to Spain and then Mexico, the loyalties of the overwhelmingly Protestant settlers remained to the United States. Grievances against the Mexican Government grew, and when it adopted a new centralist constitution and abolished slavery, an institution upon which many Texas settlers depended, a movement for succession developed. Despite U.S. neutrality laws, the movement received considerable support from American citizens in the form of money, arms, and volunteers. The Mexican Army moved to crush the Texas rebellion, but after setbacks at the Alamo and Goliad, the Texans were able to win a rousing victory at San Jacinto, capture Mexican leader Santa Anna, and gain their independence in 1836. The newly formed government of Texas promptly applied for annexation to the United States. The administration of President Martin Van Buren, however, declined since annexing Texas at that point would have meant war with Mexico, and senators of the Northern States opposed entry into the union of another slave state.
For the next 9 years Texas remained an independent republic, cultivated by England and France, who saw an independent Texas as a balance to the United States in North America. By 1845 the British convinced the Mexican Government to recognize Texan independence to forestall annexation by the United States, but it was too late. Acknowledging the idea of Manifest Destiny that had seized the American public, Secretaries of State Upshur and Calhoun negotiated a treaty of annexation with the Republic of Texas, but the treaty was rejected by the Senate due in large measure to the opposition of Northern senators. Recognizing that the two-thirds Senate vote required to ratify a treaty was unattainable, President John Tyler presented a simple declaration of the annexation of Texas to Congress, which passed it by joint resolution. The annexation led to a break in diplomatic relations with Mexico, which had never formally recognized Texas independence, and created a U.S.-Mexico boundary dispute. The historic southern border of Texas had been the Nueces River, the border recognized by the Mexican Government, while the United States recognized the Rio Grande River, the border claimed by Texas based on the treaties Santa Anna was forced to sign after his capture.