Under normal circumstances, if a lawyer reveals what a client said in confidence, or reveals strategy and analysis of a case, that information is no longer confidential, and the government can compel testimony about it. A joint defense agreement allows lawyers for people with a common interest in a case to share what they learned from clients without that information losing its confidential nature. Because everyone in the agreement has a common interest in defending the case and has agreed to keep the information secret, the theory goes, sharing the information within the group doesn’t waive its confidentiality.

Joint defense agreements are common in white-collar investigations because they allow lawyers to figure out two crucial things: what happened and what the prosecutor knows about it. Mr. Trump’s lawyers have made vigorous use of such pacts — we’ve learned he has them with Mr. Manafort and with the Roger Stone confidant Jerome Corsi.

But the pacts almost always explicitly require the parties to withdraw from the agreement immediately if they cooperate with the government. After all, once someone begins to cooperate, his interests are no longer common with the other members of the joint defense agreement — by definition, they’re now adverse. This is often how we learn that someone will soon plead guilty: The former national security adviser Michael Flynn had a defense pact with the president, but his withdrawal from it signaled his cooperation and plea.

The objectives of a cooperation agreement like the one Mr. Manafort entered with Mr. Mueller are flatly inconsistent with the obligations of a joint defense agreement. So it doesn’t matter whether or not Mr. Manafort explicitly withdrew from his deal with Mr. Trump, because once he began cooperating — or, at least, pretending to — the legal theory supporting the agreement collapsed. Because Mr. Manafort no longer had, on paper, a “common interest” with Mr. Trump, his lawyers’ communications with Mr. Trump’s lawyers could no longer be seen as cloaked with any expectation of confidentiality.

If they revealed client confidences, they waived the attorney-client privilege. Mr. Mueller can, and perhaps should, try to find out what Mr. Trump’s and Mr. Manafort’s lawyers said to one another about their respective clients after Mr. Manafort began cooperating. The resulting legal battles over privilege would be spectacular.

Some analysts speculate that Mr. Mueller intended this result — that he knew that Mr. Manafort would lie to him, knew that Mr. Manafort’s lawyers would brief Mr. Trump on those lies, and knew that Mr. Trump would foolishly repeat those lies in his written statement to Mr. Mueller, thus committing a new federal crime.

That’s a good plot for a legal thriller, but it’s not how real federal prosecutors work. Mr. Mueller is a by-the-book sort of prosecutor, not one to indulge in such ploys. To the extent he trusted Mr. Manafort and revealed details of his investigation, he made a mistake. Of course, the president’s team — never a font of shrewd criminal defense strategy — may have made a mistake, too, if they incorporated Mr. Manafort’s lies into their own written responses to Mr. Mueller’s questions. That could expand the scope of Mr. Mueller’s investigation to include new false statements to the F.B.I., the downfall of several of his targets.

Mr. Mueller’s mistake is understandable. Mr. Manafort’s lawyers’ communications with Mr. Trump’s lawyers are shocking and unprecedented, a brazen violation of criminal defense norms notable even in an investigation full of them. They are consistent with only one conclusion: Mr. Manafort and his lawyers seek a presidential pardon, not a reduced sentence through sincere cooperation.

Ken White, a former federal prosecutor, is a criminal defense lawyer and First Amendment litigator at Brown White & Osborn in Los Angeles, and a host of “All the President’s Lawyers,” on radio station KCRW.

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