The scholarship on the “Yellow Peril” looks at Japanese immigrants (Issei) as an object of anti-Asian racialization in domestic politics or as a distraction in U.S.-Japanese bilateral diplomacy. Seldom do historians consider its ramifications outside those contexts. They also lack perspective on the impact of Issei practice on the geopolitics of Yellow Peril, which spread from California to the U.S.-Mexican borderlands and beyond. This article examines the role of Issei settler colonialism, as well as its unintended consequences, in the formation of discourse on the transborder Yellow Peril. That discourse propelled white America to reaffirm its commitment to the Monroe Doctrine, shifting the nature of U.S. diplomacy from the endeavor to keep European rivals out of the Western Hemisphere to one that sought to exclude the Japanese racial enemy from America’s “backyard.” It culminated in the construction of a hemispheric national security regime.