Mexico played a relatively minor military role in World War II, but there were other opportunities for Mexico to contribute significantly. Relations between Mexico and the Unites States had been warming in the 1930s, particularly after U.S. President Franklin Delano Roosevelt implemented the Good Neighbor Policy toward Latin American countries. Even before the outbreak of hostilities between the Axis and Allied powers, Mexico aligned itself firmly with the United States, initially as a proponent of "belligerent neutrality" which the U.S. followed prior to the Attack on Pearl Harbor in December 1941. Mexico sanctioned businesses and individuals identified by the U.S. government as being supporters of the Axis powers; in August 1941, Mexico broke off economic ties with Germany, then recalled its diplomats from Germany, and closed the German consulates in Mexico. Immediately following the Japanese attack on Pearl Harbor on December 7, 1941, Mexico went on a war footing.
Mexico's biggest contributions to the war effort were in vital war materiel and labor, particularly the Bracero Program, a guest-worker program in the U.S. freeing men there to fight in the European and Pacific theaters of War. There was heavy demand for its exports, which created a degree of prosperity. A Mexican atomic scientist, José Rafael Bejarano, worked on the secret Manhattan Project that developed the atomic bomb.