Answer:
The annexation of Texas.
Step-by-step explanation:
The annexation of Texas in 1845 outraged the Mexican public and the government. After the US government and Texas concluded a treaty of annexation, Mexico severed relations with Washington. Though there was not war immediately as Mexican authorities had threatened, tensions remained high because of disputes over Texan borders. The big conflict started in 1846, and though the US army was superior in weaponry and technology, Mexicans put a stiff resistance.