Texas Annexation 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 approved by the U.S. Congress, leading to Texas becoming the 28th state. The annexation heightened tensions between the U.S. and Mexico, contributing to the outbreak of the Mexican-American War in 1846. The conflict arose partly because Mexico did not recognize Texas' independence and viewed the annexation as an act of aggression. Ultimately, the war resulted in significant territorial gains for the U.S.