what was the main cause of the u.s mexican war
Causes of the Mexican War. Leading causes of the Mexican War included: Texan Annexation. Mexico had warned it would regard annexation as an act of war. When it took place, Mexico did not declare war, but broke diplomatic relations. The Boundary Dispute. Regardless of its status (was it an American state or a rebellious Mexican province?), the United States maintained that the southern border of Texas was formed by the Rio Grande, but Mexico argued that the traditional boundary was at the Nueces River farther north.