The annexation of Texas amassed a large amount of land for the United States, but caused tensions between the United States and Mexico when finalized. Prior to annexation, Texas created disputes between Mexico and the United States. Texas gained independence from Mexico in 1836 and officially became part of the United States in 1845 under the leadership of President John Tyler, much to the dismay of Mexico.
Texas existed as a republic in 1844 and entered into statehood on December 29, 1845. Mexico, the United States and Texas disagreed on the boundaries of Texas, which caused mounting tensions upon annexation. The United States argued Texas included lands in present-day Colorado and New Mexico, and covered territory farther south to the Rio Grande River. Mexico considered Texas extending only to the Nueces River, however, lying north of the Rio Grande. Skirmishes emerged between the United States and Mexico, causing the United States' declaration of war on Mexico on May 13, 1846. Under the leadership of President James Polk, Mexico and the United States eventually settled on a boundary for Texas. The settlement occurred through the Treaty of Guadalupe-Hidalgo, which required Mexican succession of previously-owned lands and payment to the United States for debt.