I would say that Yes, the United States was and is justified for going to war against Mexico in 1846. The reason for this is because in 1884, Mexico started the War when they attacked the Army that was led by General Zachary Taylor who was order by the United States to defend the territory on the Rio Grande that was in fact claimed by the United States of America. But, it was owned as a consequence because of the treaty between Texas and Mexico when Texas first became independent.
The United States of America tried to be peaceful and negotiate the disputed territory that had once been part of Texas. The United States of America did this by sending John Slidell to Mexico City that was empowered at that time to offer Mexico substantial amounts of money for the disputed territory.
IF, big IF, the United States of America had seriously provoked the war and would have taken advantage of the fuzziness that was going on with the relevant treaties, then the people in the relevant regions which included Texas and Alta California, would have been better off under the control of America then they would if they were under control of Mexico. And the reason for this is because while this was all going on, Mexico was corrupt and confused at the time because of the Presidency changing hands rapidly and also because of the democratic traditions being abandoned along the way. AND, in some cases but very few, there were settlers that were originally Americans, had asked to be part of the United States of America!!
Now, the only basis that Mexico could claim was any superior lands right to rule that piece of land was that they had to conquer that piece of land first. Now the Spanish aristocracy of Mexico didn’t even have an “original village settler” to claim the lands. The reason for that is Mexico had already acquired them by conquering and displacing the Indian and Mesoamerican inhabitants.Â
