A huge block of new territory was acquired—approximately 525,000 square miles
The Mexican War and the tide of expansionism it unleashed underlined the political control exercised by the South in American political affairs
The addition of new lands touched off new and bitter debates on the slavery issue, as many had predicted
The Americans suffered heavy losses; the nearly 13,000 dead included only about 1,700 in combat—the rest succumbed to rampant disease
The war was a proving ground for young military officers (Jackson, Lee, Meade, Sherman, for example) who would soon put their skills to work in the American Civil War.
From the Mexican perspective:
The loss of about 50 percent of their territory was a matter of great humiliation and provoked ill feeling against the United States that has never fully dissipated
A period of political recrimination and instability followed in Mexican domestic affairs.
Many historians have regarded the Mexican War as an ignoble exercise of power fueled by the Southern slaveowners' expansionist needs.
Other observers, while acknowledging the role of the South, have noted that Mexico's hold on their northern provinces was tenuous at best. They had little control in California—an area coveted by Britain, Russia and France, as well as the United States. Texas had been independent for nine years and the other areas were largely desert with more Native Americans than Mexicans.