The United States has always been an oppressor to its neighboring countries, making any populations who stand in the way of what it wants an enemy. The Mexican-American War was a violent and life-altering event for Mexican citizens that lasted from 1846 to 1848. It dramatically altered the course of Mexican and American history for years to come. After the debilitating battle ended, the United States became a world power after acquiring more than 500,000 square miles of valuable territory and Mexico spent years recovering from the loss of its Mexican lands and citizens. Ultimately, it was "insatiable US ambition, fueled by [Mexican] weakness" that was the primary cause of the Mexican-American War. This can be broken down into many potentially feasible explanations of the root causes; including the fault of American slave owners in their support of the conquest of Mexico, the war as an American plot, and the responsibility falling on President Polk of the United States. This article examines several of these concrete theories, including Manifest Destiny, which is the belief that the United States has the right and responsibility to expand its borders outward, unresolved disputes regarding the boundaries of newly annexed Texas and the expansion of slavery. Maintaining the belief that it was destined to expand from coast to coast, this caused much conflict and heartache for citizens living in Mexico. The dispute began after the United States surrounded the Mexican nation following the Louisiana Purchase. With so much open territory available for settlers, it was natural that illegal habitation would occur. The United States "soon found itself masters of Louisiana, [ready to] extend its trap...... middle of paper...... Pletcher, David M. "The annexation completed and the push towards the Pacific.” In The Diplomacy of Annexation: Texas, Oregon, and the Mexican War, 172-226. Columbia: University of Missouri Press, 1973. Robinson, Cecil, ed. Chapultepec's View: Mexican Writers on the Mexican-American War. Tucson: University of Arizona Press, 1979.Ruiz, Ramon Eduardo, ed. The Mexican War: Was it Manifest Destiny? New York: Holt, Rinehart & Winston, 1963. Smith, Justin H. "Relations between the United States and Mexico 1825-1846, the Mexican attitude on the eve of the war and the American attitude on the eve of war.” In The War with Mexico, 58-137. Volume 1. Norwood, Massachusetts: Norwood Press, 1919. Vazquez, Josefina. “War and Peace with the United States”. In The Oxford History of Mexico, 339-69. Edited by Michael C. Meyer and William H. Beezley. New York: Oxford University Press, 2000.