Question

Why would mexican officials have been upset by the annexation of texas by the united states?

Answer 1

Mexican officials were upset by the annexation of Texas to the US because they still saw Texas as a part of Mexico, and viewed it as being robbed by the US.

Answer 2

Mexico still considered Texas to be a part of the Mexican country. Originally in 1836, Texas broke off from Mexico, but Mexico still saw it belonging to them. When the Mexican-American war ended and Mexico lost its territories to the United States the same year as the United States annexed Texas, they saw the US as robbing Mexico of its lands including Texas.