Answer: The United States was justified in going to war with Mexico. The cause of the war between the nations is the possession of Texas. Mexico's military had killed Americans on U.S. soil. Before, Mexico was an independent republic. This means it had a right to rule itself. Texas tried to become apart of the United States until Mexico interfered. Mexico tried preventing Texas from being annexed with the U.S. The United States had a right to go with war because they were protecting Texans' own choice on becoming apart of a country.