https://history.state.gov/milestones/1830-1860/texas-annexation
The Annexation of Texas, the Mexican-American War, and the Treaty of Guadalupe-Hidalgo, 1845–1848 During his tenure, U.S. President James K. Polk oversaw the greatest territorial expansion of the United States to date. Polk accomplished this through the annexation of Texas in 1845, the negotiation of the Oregon Treaty with Great Britain in 1846, and the conclusion of …