Prior to the outbreak of war with America, the Japanese used several secret diplomatic codes, the most secret of which was the exceedingly complicated cipher system known as Purple

Despite the Japanese belief that it could not be cracked, Purple was indeed broken by the Americans in August, 1940, after 18 months of intense effort in a joint US Army-Navy cooperation.

During 1939-1940, the Navy furnished the intercepts and took over breaking other Japanese diplomatic codes while the Army concentrated solely on Purple. Later, according to At Dawn we Slept, "The Army provided the solution and the wiring diagram; the Navy provided the funds and manufacturing facilities."

Thus, from the summer of 1940 onward, "US intelligence had been reading Japan's diplomatic Messages...The US government had full knowledge of virtually all the traffic which passed between the Foreign Office in Tokyo and its most important embassies and consulates abroad," as well as the lower-grade "J codes", such as the J-19 then in effect.

The decrypts had the code name of Magic, but they did not cover the Japanese Navy and Army codes. By 1941, there were eight Purple decrypt machines, but Pearl Harbor did not have one.

The Japanese did not believe that the Americans had the ability to break their code. According to the late Ladislas Farago's 1967 book The Broken Seal; "To this day, Japanese cryptologists refuse to concede that their American colleagues solved the code by actually reproducing the B unit from tell-tale clues gained from pure crypt-analysis.

"According to Noboru Kojima, 'It is assumed in Japan that a machine was stolen...' "

According to Farago, the Army initially failed to share its Magic knowledge with either President Roosevelt or the commander of its own Air Corps. General Henry H. "Hap" Arnold. He asserts that neither FDR nor Secretary of State Cordell Hull were told of the Purple/Magic secret until Jan. 23, 1941, five months after the code was cracked.

He adds that as of Dec. 6, 1941, FDR wasn't told that Pearl Harbor was in jeopardy, and that Admiral Harold Stark assured him "That the base was in no danger and that the Fleet was at sea."

In part, the was true: the carriers were at sea; only the battleships were at Pearl Harbor. "The President was never shown any of the intercepted espionage messages, and was never apprised that such reports existed at all in Tokyo," concludes Farago.

In Infamy: Pearl Harbor and Its Aftermath, author John Toland rejects all this flatly, asserting that Stark, Secretary of the Navy Frank Knox, FDR, General George Marshall and Roosevelt aide Harry Hopkins spent the night of Dec. 6th at the White House and had interpreted the latest Magic revelations correctly: "All were waiting for what they knew was coming---an attack on Pearl Harbor...

"The comedy of errors on the 6th and 7th appears incredible. It only makes sense if it was a charade, and Roosevelt and inner circle knew about the attack. A massive cover-up followed Pearl Harbor a few days later," with Marshall himself allegedly stating, 'Gentlemen, this goes to the grave with us.' "

He further believes that Hull, Secretary of War Henry L. Stimson, Hopkins and Knox all closed ranks around the President---and that General Walter Short and Admiral Husband Kimmel in Hawaii got all the blame. Many Navy men believe this as well to this day, feeling that the "professional Navy" was made the fall guy for the Pearl Harbor debacle.