"Got it?" Milley asked, according to the book.
'Milley considered it an oath,' the authors write.
"Peril" is based on more than 200 interviews with firsthand participants and witnesses, and it paints a chilling picture of 
Trump's final days in office. The book, Woodward's third on the Trump presidency, recounts behind-the-scenes moments of a commander in chief 
unhinged and explosive, yelling at senior advisers and aides as he desperately sought to cling to power.
It also includes exclusive reporting on the events leading up to January 6 and Trump's reaction to the insurrection, as well as newly revealed details about Trump's 
January 5 Oval Office showdown with his vice president, Mike Pence.
Woodward and Costa obtained documents, calendars, diaries, emails, meeting notes, transcripts and other records.
The book also examines Joe Biden's decision to run for office again the first six months of his presidency why he pushed so hard 
to get out of Afghanistan and how he really feels about Trump. CNN obtained a copy of "Peril" ahead of its release on September 21.
'You know he's crazy'
Milley's fear was based on his own observations of Trump's erratic behavior. His concern was magnified by the events of January 6 and the 'extraordinary risk' the situation posed to US national security, the authors write. Milley had already had two back-channel phone calls with China's top general, who was on high alert over the chaos in the US.
Then Milley received 
a blunt phone call from House Speaker Nancy Pelosi, according to the book. Woodward and Costa exclusively obtained a transcript of the call, during which Milley tried to reassure Pelosi that the nuclear weapons were safe.
Pelosi pushed back.
"What I'm saying to you is that if they couldn't even stop him from an assault on the Capitol, who even knows what else he may do? And is there anybody in charge at the White House who was doing anything but kissing his fat butt all over this?"
Pelosi continued, "You know he's crazy. He's been crazy for a long time."
According to Woodward and Costa, Milley responded, "Madam Speaker, I agree with you on everything."
After the call, Milley decided he had to act. He told his top service chiefs to watch everything "all the time." He called the director of the National Security Agency, Paul Nakasone, and told him, "Needles up ... keep watching, scan." And he told then-CIA Director Gina Haspel, "Aggressively watch everything, 360."
The authors write, 'Milley was overseeing the mobilization of America's national security state without the knowledge of the American people or the rest of the world.'
Woodward and Costa also write that 'some might contend that Milley had overstepped his authority and taken extraordinary power for himself,' but he believed his actions were 'a good faith precaution to ensure there was no historic rupture in the international order, no accidental war with China or others, and no use of nuclear weapons.'
Trump going rogue
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.'
Milley called it the 'absolute darkest moment of theoretical possibility,' the authors write.
"Peril" is one of several books released this year that have documented the tumultuous final days of Trump's presidency. In "I Alone Can Fix It," Washington Post reporters Phil Rucker and Carol Leonnig detailed how Milley 
discussed a plan with the Joint Chiefs to resist potential illegal orders from Trump amid fears that he or his allies might attempt a coup.
'Wag the Dog'
Woodward and Costa write that top national security officials were worried Trump might pull a "Wag the Dog" - - provoking a conflict domestically or abroad to distract from his crushing election loss.
When Trump 
refused to concede in November 2020, Haspel warned Milley, "We are on the way to a right-wing coup. The whole thing is insanity. He is acting out like a six-year-old with a tantrum." Haspel also worried that Trump would try 
to attack Iran.
"This is a highly dangerous situation. We are going to lash out for his ego?" she asked Milley, according to the book.