You can find a number of excerpts from the Woodward and Costa book at CNN including this nightmare.

Milley’s fear that Trump could do something unpredictable came from experience. Right after Trump lost the election, Milley discovered the President had signed a military order to withdraw all troops from Afghanistan by January 15, 2021, before he left the White House.

The memo had been secretly drafted by two Trump loyalists. No one on the national security team knew about it, according to the book. The memo was eventually nullified, but Milley could not forget that Trump had done an end run around his top military advisers.

Woodward and Costa write that after January 6, Milley ‘felt no absolute certainty that the military could control or trust Trump and believed it was his job as the senior military officer to think the unthinkable and take any and all necessary precautions.’