The Mexican-American War (1846-1848) was caused by many factors. Going back to 1836, one of the reasons was the Treaty of Velasco. The secret treaty had called for recognition of the Rio Grande as the border, and the release of Santa Anna.
Explanation:
the Nueces River by Taylor's troops as an act of war so Mexican troops were ordered to cross the Rio Grande. President Polk saw this as aggression towards the US and he declared war on Mexico on May 13, 1846 with the vote of 173-14 in Congress and 42-2 in the Senate both in favor of the war,2 3thus starting the Mexican-American War.
