The official motivation was to outmaneuver suspected diplomatic efforts by the British government for 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.
The Annexation of Texas, the Mexican-American War, and the Treaty of Guadalupe-Hidalgo, 1845–1848. ... Following Texas' successful war of independence against Mexico in 1836, President Martin van Buren refrained from annexing Texas after the Mexicans threatened war.
