'''3.''' The Mexican War started when President Polk told Congress that an American army had been attacked by Mexicans on American land. That prompted Congress to retaliate by declaring war with Mexico, based on a fact that is dubiously debated today. The war was ended when America took Mexico City in the September of 1847. | '''3.''' The Mexican War started when President Polk told Congress that an American army had been attacked by Mexicans on American land. That prompted Congress to retaliate by declaring war with Mexico, based on a fact that is dubiously debated today. The war was ended when America took Mexico City in the September of 1847. |