This article examines four decades of anti-Japanese paranoia in popular American media, particularly in California, from the early 1900s to the eve of Japan's attack on Pearl Harbor. It illustrates the overlooked influence that this hysteria had in shaping American perceptions of Japanese immigrants in Baja California, Mexico, and the consequences of those views for these borderlands prior to 1941. Drawing on California and U.S. national newspapers, contemporary novels, and U.S. government records, the article shows that the presence of Japanese immigrants in Baja California was for decades used as a pretense by American interest groups seeking to annex the peninsula. Beneath these alleged security concerns were strong economic interests, among which obtaining sole control over the Colorado River figured prominently. Decades of annexation calls based on a supposed Japanese threat, this article argues, influenced the Mexican government's 1942 decision to place its citizens of Japanese descent in internment camps.