The Texas annexation was the 1845 annexation of the Republic of Texas into the United States of America. Texas was 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.
Texas was annexed by the United States in 1845 and became the 28th state. Until 1836, Texas had been part of Mexico, but in that year a group of settlers from the United States who lived in Mexican Texas declared independence. ... The annexation of Texas contributed to the coming of the Mexican-American War (1846-1848).
The annexation question became one of the most controversial issues in American politics in the late 1830s and early 1840s. The issue was not Texas but slavery. ... At this point, pro-slavery Southerners began to popularize a conspiracy theory that would eventually bring Texas into the Union as a slave state.