The event that triggered the start of the Mexican-American War was the annexation of Texas by the United States in 1845, which Mexico still considered part of its territory following Texas' independence in 1836. Tensions escalated when President James K. Polk sought to expand U.S. territory further west, leading to disputes over the Rio Grande and Nueces River boundaries. The situation intensified when U.S. troops were sent to the disputed area, resulting in clashes with Mexican forces. The conflict officially began in April 1846 and concluded with the Treaty of Guadalupe Hidalgo in 1848, significantly altering the territorial landscape of North America.