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.
