The December 4, 1941 Chicago Tribune, New York Daily News, and Washington Times Herald each ran a front-page story headlined "F.D.R.'s War Plans" authored by Chesly Manly, the Tribune's Washington correspondent. Manly's story claimed to be based on actual war plans drawn up by the Army and Navy for involvement in WWII, which included the proposed establishment of a 10 million man military force of which about half would be devoted to the war in Europe. The leak, appearing in prominent anti-Roosevelt newspapers, undercut Roosevelt's claims he had no plans to involve the country in the war.

Those familiar with actual joint warplan known as Rainbow 5 immediately realized the article was directly based on those plans, meaning a leak had occurred. Roosevelt was furious and demanded to know how the leak had occurred.

Investigations launched did not reveal the leaker. Manly refused to reveal his source, and Attorney General Francis Biddle refused to allow the FBI to attempt to compel disclosure. Some cooperative journalists told the FBI that Manly had probably received the source from an isolationist senator, with some identifying David I. Walsh (D-MA, in office 1919-25, 1926-47). (This link did not go anywhere, but Walsh--a lifelong bachelor--was soon caught up in a scandal involving a Brooklyn male bordello infiltrated by Nazi spies.) But Rainbow 5 had never been given to Congress, so any politician had to have a source in the armed forces or in the War or Navy Departments.

Within the armed forces and their associated departments, investigation started with Major Larry Kuter, assistant secretary of the War Department General Staff who was charged with security of the plans. Kuter could account for all copies except the five that were in the custody of senior officers. Kuter was cleared and thanks to very rapid promotions recommended by Army Chief of Staff George C. Marshall himself was a brigadier general in February 1942. Kuter retired in 1962 at the rank of general.

Suspicion swirled around Lt. Col. Albert C. Wedemeyer, one of the chief authors of the plans and suspected supporter of the America First isolationist movement. Wedemeyer proclaimed his innocence and was also cleared. He became something of an Asia specialist, remained involved in strategic planning, and retired as a lieutenant general (being promoted by Congress to general after retirement). In his own memoir, Wedemeyer Reports! (1958), the general described himself as the "planner of a war I did not want." Wedemeyer also described the FBI investigation, whose file he later read.

It turned out the leak really did go through an isolationist in Congress. In 1962, Senator Burton K. Wheeler (D-MT, in office 1923-1947) published his autobiography, Yankee from the West: The Candid Story of the Freewheeling U.S. Senator from Montana, in whose first chapter he confessed to being Manly's source.

Although a progressive Democrat and generally a supporter of Roosevelt's New Deal (though an opponent of the court-packing plan), Wheeler was vehemently opposed to any involvement in foreign wars and was also a prominent America Firster himself, though like most members of that faction he became a supporter of the war after Pearl Harbor.

Wheeler said he had received Rainbow 5 in a brown envelope from an Army Air Forces captain who came to his home. Wheeler believed the captain be the emissary of a high-ranking officer who desired certain facts to be known. Wheeler's son later said the senator had specified he believed the officer was Army Air Forces chief Henry H. "Hap" Arnold. Wheeler said the captain had previously spoken to him about problems with Army aircraft as well as way Roosevelt was leading America into the war without admitting to the public what he was doing. Wheeler asked the captain why he was giving him "the most closely guarded secret in Washington" and received this answer:

"Congress is a branch of the government," he replied. "I think it has a right to know what's really going on in the executive branch when it concerns human lives."

Wheeler never identified the captain by name. Upon receiving the report from the captain, Wheeler read it and decided the public needed to know what it said. He feared his fellow senators would bury it. So he called Manly, and the two took turns reading portions of Rainbow 5 to Wheeler's secretary who copied in shorthand. The report was then returned to the captain who put it back where it belonged.

Interestingly, from his reading of the FBI file, Wedemeyer reports that in 1942 Assistant Secretary of War John J. McCloy had told the investigators that he believed an isolationist in Congress had leaked the report. Wedemeyer also reports an Air Force major told investigators he believed Rainbow 5 was a "plant," meaning it was not a serious warplan but was instead intended to be leaked so that the Axis and American public knew the country was preparing for war.

Unless we think Wheeler was just lying, we can be pretty sure of the anonymous captain to Wheeler to Manly part of the story. But did anybody send the captain? Kuter's custodianship of the documents suggests a senior officer was involved, though I suppose the possibility of an officer acting on conscience can't be discounted.

Wheeler seems to have thought Arnold sent the captain, though he held back from saying this in print. FBI sources later revealed that Arnold was suspected of being the senior officer behind the leak and that the investigation wound up once this was discovered. Certainly Arnold had access to the plan as well as a supply of junior officers to act as bagmen. Arnold's potential motive can only be guessed at. Bureaucratic leaks are sometimes made in order to gain support for policies. Wheeler and some of his supporters suggest Arnold wanted to get the public more interested in airpower. But it's hard to make a direct connection between the leak and any particular conclusion on that point. Arnold and his staff had significant input into Rainbow 5 and it included a prominent role for the Air Force. Arnold died in 1950, before the significant disclosures were made.

In his well-known true espionage book A Man Called Intrepid (1976), William Stevenson suggested eponymous British spy William Stephenson (no relation to the author) orchestrated the plan in order to trick Germany into declaring war on the United States and that he managed to send the unknown captain. That book, while a fun read, is generally dismissed by historians.

Late in life, Wedemeyer himself believed, admittedly without evidence, that Roosevelt himself had engineered the leak. Historian Thomas Fleming agreed, setting forward this theory in an article entitled "The Big Leak" in the December 1987 issue of American Heritage. Fleming argues Roosevelt's motive was basically that credited to Intrepid: make sure that America entered the war against Germany as well as Japan, the latter of which he knew was imminent in late 1941 given the breakdown in negotiations. (Fleming admits other historians he consulted disagreed with his theory, including Forrest Pogue who believed Marshall would surely have told him about it.)