Manifest Destiny was the 19th century belief that America had a God-given destiny to expand its territory from coast to coast. Advocates argued the US had a responsibility to gain valuable natural resources and civilize the land. The document outlines the annexation of Texas from Mexico, including key events like the Battle of the Alamo and the Texas Revolution. It led to the Mexican-American War, where the US gained additional territory in the Southwest through the Treaty of Guadalupe Hidalgo. While expansionism was controversial due to slavery issues, the principle of Manifest Destiny ultimately prevailed in bringing Texas into the union as a state.