Gen. Milley should be asked to clarify, under oath, the context of his communications with China and nuclear launch procedure when he testifies before the Senate on Sept. 28. America’s military brass rightly has deconfliction channels open with adversaries when their forces are in proximity, but promising a tip off before the President ordered an attack would be an outrageous usurpation.

While the military reviewing “long-established” nuclear protocols is hardly a scandal, the book suggests this was done after his calls with Mrs. Pelosi. Generals can take her calls, but she’s not in the chain of command.

Mr. Trump was erratic in the final days of his term, staging an unprecedented if doomed political effort to overturn an American election. But if Gen. Milley genuinely felt the President was that much of a global menace, he should have sounded the alarm and resigned. Figures like William Barr and Don McGahn constrained Mr. Trump’s worst instincts without eviscerating political norms.

Chairmen of the Joint Chiefs are typically respected across the political aisle. Gen. Milley’s reputation is already damaged by a botched Afghanistan withdrawal and a Kabul drone strike that killed civilians and is still unexplained. Even if Mr. Biden retains confidence in Gen. Milley, as he said Wednesday, the general’s credibility is in doubt. If the book’s account isn’t accurate, he needs to say so explicitly and specifically.