When the Mexican-American War began in 1846, feelings about it were greatly mixed in the United States. There were many who supported the war. This faction was led by President James Polk and congressional Democrats. They argued that Mexican incursions into the newly acquired, yet disputed, Texas territory should be considered a direct attack on the United States. They also argued that the sparsely settled territories of California and New Mexico had too small a Mexican population to actually be considered properly Mexican. They made the argument that leaving these territories in Mexican hands left them open to annexation by the British. Furthermore, it was becoming clear that Mexico was unable to repay its debts to the United States. Supporters of the war argued that Mexico should thus pay it back in the form of territory. Overall, the Americans who supported the war saw it as a way to fulfill the promise of Manifest Destiny.
Many in the United States opposed the war. The anti-slavery faction feared that adding new territory would mean more opportunity for the establishment of new slave states. This would upset the delicate balance that had been maintained between slave and free interests for decades. This cause was led by Massachusetts congressional representative and former president John Quincy Adams.
Mexicans unilaterally saw the war as imperial aggression on the part of their neighbor to the north. To Mexicans, this event was considered more of an invasion than a war. They saw the previous annexation of Texas as a violation of international law and the theft of a major territory. When the United States moved to take the New Mexican and Californian territories, Mexicans had no choice but to attempt to fight off the invaders.