Originally Posted by Captain America
For the OP;
The Mexican-American War was unjust;
It should be mentioned (again) that Mexico had no more populated most of that country than the French occupied most of the Louisiana Purchase. Many of those areas were already largely populated by Americans, not by Mexicans. In many of those places, Mexicans dared not go because they were controlled by hostile Indian tribes. The Mexican claim to most of it was only nominal.
Further, a good part of California was variously claimed by Mexico, Britain and Russia. Since none of them had actually occupied a significant part of it, none had a better claim to it than did any other, and Americans rightly had a better claim to some of it than anyone because they actually occupied it.
It's a close question just who actually started the Mexican War, by the way.