Your essay needs to say whether or not you believe the Mexican-American War was justified, and then you need to back up your opinion with reasons/evidence/details.
In my opinion, the Mexican-American War was not justified. The annexation of Texas by the United States, which was a major cause of the war, was done in disregard of previous agreements between Mexico and Texas. Additionally, the dispute over the Texas-Mexico border was fueled by the expansionist desires of the United States, leading to an unnecessary conflict. The war resulted in significant loss of life and territory for Mexico, further highlighting its unjust nature.
