Cracking the enemy’s code can make make all the difference, but the achievement must always be one of the nation’s best kept secrets. That was true in World War II, when American cryptographers broke the Japanese code, learned Japanese naval plans, and led to a decisive victory in the Battle of Midway, turning the tide of the Pacific war.

Likewise, when the United States cracked the Iranian code, it could monitor Iran’s efforts to develop nuclear weapons and assistance it was giving to anti-American insurgents in Iraq. So it came as a nasty shock when American officials learned that their formerly trusted ally, Ahmad Chalabi, had informed the Iranian government that the United States had broken the secret communications code of its intelligence service.

When the story started to leak, American officials gave out details. About six weeks ago, Mr. Chalabi told an Iranian official in Baghdad that the Americans had broken the code. The Iranian officials evidently didn’t believe Mr. Chalabi and took a chance of using the same code to tell Tehran what Mr. Chalabi had said. The message quoted Mr. Chalabi as having said that “one of them” – meaning the Americans – had disclosed in a drunken conversation, that the United States was reading its coded messages. In fact, they were, and when they decrypted that message they knew that Mr. Chalabi had given away the vital secret.

This betrayal seems to have been the last straw that ended the deteriorating U.S. relationship with Mr. Chalabi. The Pentagon cut off the $340,000 monthly payments it had been giving him for intelligence assistance. And American and Iraqi troops ransacked his palace in Baghdad and confiscated documents and communications equipment. Pentagon investigators now are questioning anyone who knew Mr. Chalabi and might have learned that the code had been broken. A prime suspect would be some American who liked to drink with Mr. Chalabi.

Don’t count out this wily manipulator just yet. He denies vehemently that he ever knew any American secrets, let alone gave them away. Some of his friends in the Pentagon still stand by him, arguing that, for all his faults, he did tip them to four of Saddam’s henchmen and this aided in their capture. One of Mr. Chalabi’s key contacts and backers, Richard Perle, former chairman of the Defense Policy Board, makes a good point that it made no sense for the Iranian official in Baghdad to continue using the code that had already been broken.

Still, Mr. Chalabi has a long history of self-promotion and deception. The Iraqi defectors that he introduced to U.S. intelligence officials and certain newspaper reporters spread distorted and fabricated information. A prime example is their account of mobile labs supposedly used to produce forbidden chemical or biological weapons. Thorough investigation has now concluded that the trailers were used merely for Iraqi weather balloons. But the false information found its way into newspaper stories and statements by President Bush and Vice President Dick Cheney.

Mr. Chalabi seems to have a point when he says all he did was help furnish information at U.S. request and that the CIA should have checked it before using it. The basic truth seems to be that U.S. promoters of the Iraq war used Mr. Chalabi and his informants to make their case that Saddam Hussein had weapons of mass destruction, had figured in the Sept. 11 attacks, and constituted an imminent threat that justified the war. Mr. Chalabi became their tool.