Was the Mexican War justified?
The answer to this depends largely on whose ethical standards you apply. To us today, this war was utterly unjustified and our country would never embark upon such a war today. However, by the standards of its time, this war was much more justifiable.
Today, we are very concerned with the idea that there must be a noble purpose to our wars. We go to war to make the world safe for democracy or to prevent the spread of communism or to end terrorism. By these standards, there was no reason to go to war with Mexico. Mexico posed no threat to the United States. It did not have an evil ideology that it was hoping to spread. There was no noble cause and no serious issue of self-defense. Therefore, we would clearly say today that this war was not justified.
In times past, however, the idea of a war to take territory and to make one’s country stronger was much more acceptable than it is today. The idea of Manifest Destiny made it clear that the United States deserved the territory more than Mexico did. Taking the territory would make the US more powerful as well. With these sorts of justifications in hand, it was acceptable to use things like the snub of Slidell and the attack on American forces in the disputed area between the Rio Grande and the Nueces River as pretexts for war.
Thus, this war was perfectly justified when it happened, but it would be seen as a horribly unjust war today.