What are the differing viewpoints of Americans and Mexicans on the Mexican-American War?
Americans viewed conflict as a necessity to expand westward, due to the Manifest Destiny: a belief proposing continental expansion of the United States. This belief however, was actively promoted by slave owners who wanted to increase their power - Polk was a North Carolinian native, and raised in Tennessee. After the war, both Texans and New Mexicans were actively targeting Native Americans to expand such dark endeavor. This way of thinking however, was unsustainable and eventually led to the American Civil War (1861-1865).
For Mexicans, it was a war provoked by the United States, who annexed Texas - a "rebel province" that had split-up from Mexico after the Texas Revolution (1835-1836). Mexico had warned the US that annexation would mean war, and it ultimately did, even if they didn't start it: due to skirmishes among Texans and Mexicans in a no man's land between the Nueces and Rio Grande rivers, the conflict escalated and was used by Polk as a pretext to start the war.
Nowadays, this war is considered the foundation of what later would be the United States as a superpower: without California and Texas, the United States may have ended up as another Canada. For Mexico, it was the beginning of a series of wars that undermined the country's economic vitality for almost a century and resulted in the struggling third world country it is today.