Ladies and gentlemen, the bloggy stylings of Kathy Olmsted, who brings you another edition of “On This Day in History.”

On the evening of December 6, 1941, a Navy lieutenant carried a copy a telegram to President Franklin Roosevelt, who was working in his study with his aide Harry Hopkins. This telegram was the product of one of Washington’s most tightly held secrets: the intercepted, decrypted messages sent between Tokyo and its diplomats in Washington, D.C., codenamed “Magic.” In this particular message, Tokyo told its Washington diplomats to prepare to destroy their codebooks and cipher machines. Roosevelt took about ten minutes to read the sheaf of fifteen typed pages, and then turned to Hopkins. “This means war,” the president said, essentially, and Hopkins agreed.

But the U.S. cryptogaphers never decoded a Japanese message saying “we will attack Pearl Harbor.” All of the U.S. leaders expected an attack on the Philippines, not Oahu. Several top officials began nonstop meetings on Sunday morning, December 7, as they figured out how to respond to a virtually certain Japanese attack. But no one tried to alert the Pearl commanders until less than an hour before the attack, when General George Marshall shunned the insecure telephone in favor of Western Union. The messenger with the crucial warning cable delivered it to Army Commander General Short’s office after the attack was over.

Roosevelt and his aides never gave the information about the decoded telegrams to the Roberts Commission, which investigated Pearl Harbor, for the very good reason that the Japanese had not changed their codes and the U.S. was still gleaning key information from their secret messages, and probably for the less justifiable reason that their failure to imagine a Japanese attack on Hawaii was, well, embarrassing.

A similar intelligence failure occurred on September 11, 2001. I won’t even get into the CIA’s failure to alert the FBI that well-known terrorists were in the country, or the FBI’s failure to act on the Phoenix or Minneapolis memos. Let’s just talk about the presidential failure. The CIA had warned President Bush a month before the 9/11 attacks that bin Laden was poised to strike in the United States, perhaps by directing his followers to hijack airplanes. “Bin Laden determined to strike in U.S.,” read the clear headline on the memo.

Yet the U.S. government took none of the basic actions – such as bolting shut cockpit doors – that could have prevented the tragedy. Then-national security adviser Condoleezza Rice insisted that “I don’t think anybody could have predicted that these people would take an airplane and slam it into the World Trade Center, take another one and slam it into the Pentagon; that they would try to use an airplane as a missile, a hijacked airplane as a missile.” She later had to admit that “somebody did imagine it” – in fact U.S. intelligence agents had warned of terrorists planning to use hijacked planes as missiles at least a dozen times in recent years.

In both of these cases, intelligence failures led to catastrophe – and the cover-ups of these failures led to conspiracy theories. The Pearl Harbor theories began during the war itself, sponsored by the Roosevelt-hating Chicago Tribune. For years afterward, the Pearl Harbor conspiracy theories were the province of the anti-New Deal right. Journalist John T. Flynn, who said he thanked God for Joe McCarthy, and Harry Elmer Barnes, hero to holocaust deniers, were the leading advocates of the theories.

In the 1970s and 1980s, the collapse of faith in the government after Vietnam and Watergate inspired a new generation of Pearl Harbor conspiracy books. But none of these books ever proved the central allegation: that Roosevelt had known in advance the specific location of the Japanese attack.

The lesson to draw from all this: Roosevelt had some reasons beyond his own embarrassment to keep the secrets about Pearl Harbor. But Bush and his people were just covering up for their own embarrassing failures.

For good discussions of the use and abuse of the memory of Pearl Harbor, see Emily Rosenberg, A Date Which Will Live, and Martin Melosi, The Shadow of Pearl Harbor.