The United States has always been an oppressor of neighboring countries, making enemies of all populations that stand in the way of what they want. The War between the United States and Mexico was a violent and shocking event for Mexican citizens that lasted from 1846 to 1848. It dramatically changed the course of Mexican and American history for years to come. Once the debilitating battle was over, the United States emerged as a world power having acquired more than 500,000 square miles of valuable territory, and Mexico spent years recovering from the loss of Mexican lands and citizens. Ultimately, it was “the insatiable ambition of the United States, abetted by [Mexican] weakness” that was the root cause of the U.S.-Mexico War. This can be broken down into many potentially feasible explanations about the root causes; including the guilt of American slaveholders in their support of the conquest of Mexico, the war as an American conspiracy, and the responsibility of President Polk of the United States. This article discusses many 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, the unresolved disputes regarding the borders of newly annexed Texas, and the expansion of slavery. maintained the belief that it was destined to expand from ocean to ocean, caused great conflict and suffering to the citizens living in Mexico. The dispute first began after the United States surrounded the nation of Mexico following the Louisiana Purchase. With so much open land available to settlers, it was natural that illegal settlements would occur. The United States “soon saw itself as master of Louisiana, [ready to] lay its trap… in the middle of the paper… Pletcher, David M. “Annexation Completed and The Push to the Pacific.” In The Diplomacy of Annexation: Texas, Oregon, and the Mexican War, 172-226. Columbia: University of Missouri Press, 1973. Robinson, Cecil, ed. The View from Chapultepec: 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. "United States-Mexico Relations 1825-1846, Mexican Attitude on the Eve of War, and American Attitude on the Eve of War." In the war with Mexico, 58-137. Volume 1. Norwood, Mass.: Norwood Press, 1919.Vazquez, Josefina. “War and Peace with the United States.” In Oxford History of Mexico, 339-69. Edited by Michael C. Meyer and William H. Beezley. New York: Oxford University Press, 2000.