The idea that Americans had the right to expand became known as Manifest Destiny that first appeared in print in 1845, but had been popular for decades prior. The idea was that American's "manifest desitiny [was] to overspread the continent allotted by Providence for the free development of our multiplying millions." In other words, God granted Americans the right to move West and take whatever land possible. This was echoed in President Polk's Innagural Address in 1844, in which he put forth the idea that America was destined to expand democratic institutions, and that this was a moral right. "It is confidently believed that our system may be safely extended to the utmost bounds of our territorial limits, and that as it shall be extended to bonds of our Union, so far from being weakened, will become stronger" (Manifest Destiny, 2005). Pressure built so much and there were so many Americans in Texas that in December of 1845, Texas became the 28th State of the Union, fully annexed by the United States. Mexico did not recognize this, still believing that Texas was a rebellious republic. Internally, despite U.S. offers of purchasing Texas, New Mexico, Arizona and California,...

This was exacerbated by President Polk ordering forces into disputed territory, specifically the Nueces River, and creating a fort on what Mexico considered Mexican terriroty. Thus, between what Mexico perceived as an illigeal invasion by American forces and an illegal annexation of Mexican territory, the Mexican Army began a campaign against American fortifications. The war was fought on several fronts and, combined with an extremely high rate of desertion from the Mexican army and internal political trouble in Mexico City, one of the United States' first major victories against a supposed superior foreign power (The U.S.-Mexican War, 2004).
Works Cited

The U.S.-Mexican War. (2004, March). Retrieved from dmwv.org: http://www.dmwv.org/mexwar/mexwar1.htm

Manifest Destiny. (2005, March). Retrieved from U.S. History.com: http://www.u-s-history.com/pages/h337.html

Eisenhower, J. (2000). So Far From God: The U.S. War With Mexico, 1846-48. New York: Random House.

Feldman, R. (2004). The Mexican-American War. Minneapolis, MN: Lerner Publishing Company.