How was the US expansion justifiable in the 1800s give three reasons that are both justifiable and unjustifiable next question how was war treaties and settlement justifiable or unjustifiable in the US expansion

This is of course somewhat of a subjective question, but in general most would agree that in general expansion was not justifiable since the Mexicans and Natives were doing nothing to provoke the US. One could argue it was justifiable since Americans needed more land.