Answer: There were multiple events leading to this war. Shortly after Texas becoming a state in 1845, Mexico breaks diplomatic relations with the U.S. The American idea of Manifest Destiny caused some attempts from the U.S to purchase California and New Mexico, where Mexico refuses. This eventually leads to the President sending military forces, which leads up to the Mexican-American War.
The relationship was not good because they disagreed about where the Texas- Mexico border.
The war—in which U.S. forces were consistently victorious—resulted in the United States' acquisition of more than 500,000 square miles (1,300,000 square km) of Mexican territory extending westward from the Rio Grande to the Pacific Ocean
