attack, and it was confirmed by the Japanese Navy staff only three weeks before Pearl Harbor. In the 1920s, the decisive battle was supposed to happen near the Ryukyu islands; in 1940 it was expected to occur in the central Pacific, near the Marshall islands. War Plan Orange reflected this in its own planning for an advance across the Pacific. Yamamoto's decision to shift the focus of the confrontation with the U.S. as far east as Pearl Harbor, and to use his aircraft carriers to cripple the American battleships, was a radical enough departure from previous doctrine to leave analysts in the dark. There had been a specific claim of a plan for an attack on Pearl Harbor from the Peruvian Ambassador to Japan in early 1941. (The source of this intelligence was traced to the Ambassador's Japanese cook. It was treated with skepticism, and properly so, given the nascent state of planning for the attack at the time and the unreliability of the source.) Since Yamamoto had not yet decided to even argue for an attack on Pearl Harbor, discounting Ambassador Grew's report to Washington in early 1941 was quite sensible. Later reports from a Korean labor organization also seem to have been regarded as unlikely, though they may have had better grounding in actual IJN actions. In August 1941, British Intelligence, MI6, dispatched its agent Duško Popov, code name Tricycle, to Washington to alert the FBI about German requests for detailed intelligence about defenses at Pearl Harbor, indicating that the request had come from Japan. Popov further revealed that the Japanese had requested detailed information about the British attack on the Italian fleet at Taranto. For whatever reason, the FBI took no action. British advance knowledge and withholding claims Several authors have controversially claimed that Winston Churchill had significant advance knowledge of the attack on Pearl Harbor but intentionally chose not to share this information with the Americans in order to secure their participation in the war. These authors allege that Churchill knew that the Japanese were planning an imminent attack against the United States by mid-November 1941. They furthermore claim that Churchill knew that the Japanese fleet was leaving port on November 26, 1941 to an unknown destination. Finally, they claim that on December 2, British intelligence intercepted Admiral Yamamoto's signal indicating December 7 as the day of an attack. One story from author Constantine Fitzgibbon claimed