In 1845, Texas was annexed by the United States as part of the expansionist movement later known as Manifest Destiny. Texas had declared independence from Mexico in 1836 and operated as the independent Republic of Texas for nearly a decade before joining the Union.
The annexation was politically contentious because Texas permitted slavery, and adding it as a state threatened to upset the balance between free and slave states. Presidents Andrew Jackson and Martin Van Buren had declined to annex Texas partly because of this controversy and the risk of provoking Mexico.
Congress approved annexation shortly before James K. Polk took office, and Texas formally became a U.S. state on December 29, 1845. Mexico still considered Texas part of its territory, and disputes over the southern boundary helped lead to the Mexican-American War in 1846.
The phrase “Manifest Destiny” was popularized in 1845 by journalist John L. O’Sullivan. Supporters used it to argue that the United States had a providential mission to expand across North America, though the idea was strongly opposed by anti-slavery politicians and others who condemned expansion and its consequences for Indigenous peoples and neighboring countries.