From 1915 to 1917 the anarchist-inspired Plan of San Diego (named after the small town in South Texas where it was devised) sought to redress the suffering of some ethnic poor in America by creating two new, independent republics from states in the Southwest and the Great Plains where Hispanics, blacks, Native Americans, and Japanese could live free from "capitalist oppression." The plan proposed liberating first, as a Spanish-speaking homeland, the lands Mexico had lost to the United States in 1848, namely Texas, New Mexico, Arizona, Colorado, and California. Then six bordering states, Nevada, Utah, Wyoming, Nebraska, Kansas, and Oklahoma, would be freed and given to blacks and Indians (Japanese could live anywhere). These goals would be achieved by killing all white males over the age of sixteen and would begin with assaults against South Texas.

The plan's ideology derived from the Mexican- origin, Spanish-language anarchist newspaper Regeneración, which drew 40 percent of its subscribers from the Great Plains even though it was published in Los Angeles by Ricardo and Enrique Flores Magón. Both men were convicted of violating United States neutrality statutes, in part by inciting the Plan of San Diego. Raiding against the United States from Mexico intensified to the point of contributing to the war crisis between the two countries in the summer of 1916. Early in 1917, when President Woodrow Wilson learned from the intercepted Zimmermann Telegram of a German proposal to return to Mexico lands previously lost to the United States, a proposal similar to that of the Plan of San Diego, it played a role in Wilson's decision to declare war on Germany.

A combination of military and political actions by the United States and Mexico against plan insurgents on both sides of the border defused the movement in 1917. Such success, however, was little appreciated at the time, and the legacy of the Plan of San Diego embittered United States–Mexico relations for seventy years.