Read this passage from PBS:
Despite arguments over whether the war was right, Americans had tremendous success on the battlefield. Young officers like [Ulysses S.] Grant and Robert E. Lee, who would later lead armies against one another in the Civil War, had their first combat experiences in Mexico...
Although the Mexican War had been won, the conflict over what to do with the vast amounts of territory gained from the war sparked further controversy in the U.S. The question over whether slavery would spread to these new territories would drive North and South even further apart.
Cite this source: (PBS.org)
Choose one quote from the passage above that shows an effect of the Mexican-American War.