By 1941, an estimated 6,000 Japanese immigrants and 13,000 Mexican-born citizens of Japanese ancestry resided in Mexico. The coming of war between the United States and Japan did not immediately lead Mexico's government to declare war on Tokyo. Nonetheless, under pressure both from the United States government, who feared an Asian invasion, and from nationalists within the country, Mexico's government imposed strong measures against residents of Japanese ancestry. In mid-1941, Mexico banned export of strategic goods to Japan, and after Pearl Harbor Mexico broke off diplomatic relations with Tokyo, froze all bank accounts of Japanese nationals and imposed travel restrictions.

Even as Washington pressed Mexico's government to increase hemispheric security and American official propaganda in Latin America stressed the danger of Japanese subversion in Mexico, West Coast Mexican elites and nationalists became increasingly anxious over the spectre of local Japanese fifth columnists. At the beginning of 1942, for example, the executive committee of the Union of Mexican Railroad Workers in the western part of the country submitted a written report containing purported plans, based on "trustworthy information," for an upcoming Japanese raid against Mexico, to be made in conjunction with action by ethnic Japanese fifth columnists. "The activities of fifth columnists will be to blow up bridges and destroy roads, to prevent help from arriving from the United States," it read in part. The document urged the concentration of all Germans, Italians and Japanese living in the western part of the country into camps to be established in Yucatan or near the American border, where they could be closely watched. [2] Meanwhile, in mid-February Colonel Loaixa, governor of Sinaloa, insisted that Japanese agents, acting in the guise of fisherman, had mined Mexico's territorial waters. Although this was hotly denied by General Garay, the Mexican Army's chief of staff, it contributed to the larger belief that Japan was organizing an invasion of Mexico preparatory to a direct attack on the United States. [3]

Mexico, unlike other Latin American nations, repeatedly refused to surrender any of its Japanese residents for internment in the United States, on grounds that to do so would violate Mexican sovereignty. However, on January 5, 1942—with the approval, if not the direction, of Washington—the government ordered all residents of Japanese ancestry in Baja California to leave the state. Some 2,800 Mexican Japanese were forced to fill out "voluntary relocation applications" and move at least 200 miles from the coastal area and 100 miles from the US border. They were required to move at their own expense, and given only ten days to settle their affairs—only a few of their Mexican wives and mixed-race children were permitted to stay. Their homes and businesses were taken over by local Mexicans. Soon, more immigrants and their children, chased out of Sonora by government authorities, came to join them. (Although two prominent Nisei merchants in the border community of Nogales, Luis Tanamichi and Ignacio Koba, obtained court injunctions preventing their removal, they were forcibly removed as a "protective measure" in November 1942). In early March the area around Manzanillo, in the state of Colima, was emptied of Japanese. In some areas, such Sinaloa and Chiapas, powerful local officials interceded on behalf of ethnic Japanese residents and forestalled their expulsion—there were rumors of bribes placed by local communities for such protection. Tomas Valles, state treasurer of Chihuahua, exiled 57 Japanese residents to his ranch, where they spent several months as forced laborers before being released.