Pearl Harbor advance-knowledge conspiracy theory

The Pearl Harbor advance-knowledge conspiracy theory is an unproven conspiracy theory alleging that U.S. government officials had advance knowledge of Japan's 1941 attack on Pearl Harbor.

Starting from shortly after the attack, there has been debate as to what extent the United States was caught off guard, and how much and when American officials knew of Japanese plans for an attack. Several writers, including journalist Robert Stinnett, retired U.S. Navy Rear Admiral Robert Alfred Theobald, and Harry Elmer Barnes, have argued that various parties high in the governments of the United States and the United Kingdom knew of the attack in advance and may even have let it happen or encouraged it in order to ensure America’s entry into the European theater of World War II via a Japanese–American war started at "the back door", despite the fact Germany and Italy were not obliged to assist Japan in the event of aggression against another power.

The Pearl Harbor advance-knowledge conspiracy theory is rejected by most historians as a fringe theory, citing several key discrepancies and reliance on dubious sources.