Question

why did the us declare war with mexico?

Answer 1

If this is about the Mexican-American War that started in 1846, in a nutshell, it was an incident over the border of Texas that sparked it. At the time, the US saw the border of Texas as far as the Rio Grande. Mexico saw it as being further north at the Nueces River. Both sides refused to back down.

Mexico sent a cavalry force north of the Rio Grande into the disputed territory to enforce their claim and ran into an American patrol which they fought and defeated. President Polk viewed the action as proof of an invasion on US soil and then asked Congress to declare war on Mexico which was done.