In 1880, Karl Marx wrote his friend Friedrich Sorge to ask for an update on economic conditions in California. “California is very important for me,” Marx told the founder of America’s oldest socialist party, “because nowhere else has the upheaval most shamelessly caused by capitalist centralization taken place with such speed.” This upheaval forms the backdrop for this introduction, which takes nineteenth-century California as a point of departure for examining the concerns of Third Worlds Within: the excavation of historic interactions among communities of color, the effect of US racial capitalism upon these communities, and the rise of multiracial, interethnic mobilizations against US imperialism. In tracing stories of Black, Indigenous, Asian, and Latino communities, it offers a history that is simultaneously Black and Third World, and that combines antiracist, anti-imperialist, and anticapitalist struggles waged by nonwhite communities who come together as a class faction amid and despite their cultural differences.