Donald Trump’s behavior as a candidate and President unhinged some of America’s vital institutions, including the press and the FBI. It would be disturbing to find out that military leaders also responded to the President’s norm-breaking by betraying their institutional obligations.

That’s the implication of a report of national-security freelancing by Chairman of the Joint Chiefs of Staff Gen. Mark Milley at the end of Mr. Trump’s term. Congress needs to find out how much is true—not because of partisan demands for retribution against the general, but because even the appearance of attenuating civilian control of the military is damaging to democracy.

The Chairman of the Joint Chiefs has more explaining to do.

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