James K. Polk was a strong supporter of manifest destiny, the belief that the United States was destined to expand its territory across the continent. He believed that the United States had a right to the Texas annexed to the United States in 1845.
Texas
The annexation of Texas was one of the main causes of the Mexican-American War. Mexico considered the annexation of Texas an act of war and severed diplomatic relations with the United States.
The Rio Grande
The United States and Mexico disputed the border between Texas and Mexico. Mexico claimed the border was the Nueces River, while the United States claimed the border was the Rio Grande.
General Taylor's March to the Rio Grande
In 1846, President Polk ordered General Zachary Taylor to march his army to the Rio Grande. This act was seen as an invasion of Mexico and Mexico declared war on the United States.
The Mexican-American War lasted for two years and resulted in a victory for the United States. As a result of the war, the United States gained control of the vast majority of the Southwest, including the present-day states of California, Arizona, New Mexico, and Utah.
