In 1846, a dispute over the border between Texas and Mexico resulted in armed conflict, and the Mexican–American War began. ... The Mexican army attacked them. The main cause of the war was the westward expansion of the United States. All through the 19th century Americans believed it was their right to expand westward
Causes of the Mexican-American War Texas gained its independence from Mexico in 1836. Initially, the United States declined to incorporate it into the union, largely because northern political interests were against the addition of a new slave state.
