The Annexation of Texas refers to the process by which the Republic of Texas joined the United States in 1845. After gaining independence from Mexico in 1836, Texas sought to become part of the U.S. due to economic and security concerns. The annexation was controversial, as it raised issues related to slavery and territorial expansion. In March 1845, the U.S. Congress passed a resolution to annex Texas, and it officially became the 28th state on December 29, 1845. This event contributed to rising tensions between the U.S. and Mexico, ultimately leading to the Mexican-American War in 1846.