Annexation refers to the formal act of acquiring territory by a state, often through a legislative process or military force. This concept played a pivotal role in U.S. history, especially concerning the expansion of territory following conflicts, including those arising from the Mexican-American War and the subsequent Treaty of Guadalupe Hidalgo. The annexation of Texas is particularly significant as it set the stage for further territorial disputes and laid the groundwork for future U.S. expansion westward.