How did the U.S. annexation of Texas contribute to the Mexican-American War?

The U.S. annexation of Texas in 1845 was a primary catalyst for the Mexican-American War, as it fundamentally altered diplomatic relations and intensified territorial disputes between the two nations.12

The annexation contributed to the conflict in the following ways:

• Severing Diplomatic Ties: Mexico, which had never formally recognized the independence of Texas, viewed the U.S. annexation as an act of aggression. This move prompted Mexico to break off formal diplomatic relations with the United States.324
• Disputed Borders: A critical point of contention was the location of the Texas-Mexico border. While Mexico maintained that the boundary was the Nueces River, the United States claimed it was the Rio Grande, located 150 miles further south.134
• Military Escalation: U.S. President James K. Polk ordered troops to occupy the disputed territory between the two rivers. When a skirmish between Mexican and American patrols occurred in this area in April 1846, Polk used the event to secure a congressional declaration of war.324

These tensions were further exacerbated by the broader American sociopolitical doctrine of Manifest Destiny, which fueled U.S. ambitions for westward expansion.24