Before and after the assault on the US Capitol on 6 January, the most senior US general took steps to prevent Donald Trump from “going rogue” and launching a nuclear war or an attack on China, according to excerpts of an eagerly awaited new book by the Washington Post reporter Bob Woodward.

The Watergate veteran’s third Trump book, Peril, is written with Robert Costa, another Post reporter. It is due to be published next Tuesday. A week before publication day, the Post and CNN published extracts.

Following the first two books in the series, Fear and Rage, Peril covers the end of the Trump presidency and the start of Joe Biden’s first term in the White House.

Woodward and Costa portray Gen Mark Milley, the chairman of the joint chiefs of staff, as “certain that Trump had gone into a serious mental decline in the aftermath of the election”, which he insisted without evidence was fixed.

Milley reportedly thought the president had become “all but manic, screaming at officials and constructing his own alternate reality about endless election conspiracies”.

According to the authors, Milley worried that Trump could “go rogue” and told senior staff: “You never know what a president’s trigger point is.”

The Post said Milley made “a pair of secret phone calls” to his Chinese counterpart, Gen Li Zuocheng, to say Washington would not strike Beijing.

One call reportedly took place on 30 October 2020 – four days before the election. The other took place on 8 January, two days after Trump supporters attacked Congress, seeking to overturn that defeat.

In the first call, prompted also by tensions in the South China Sea and over the origins of the coronavirus pandemic, Milley reportedly told Li: “I want to assure you that the American government is stable and everything is going to be OK. We are not going to attack or conduct any kinetic operations against you.”

Milley also reportedly told the Chinese general he would warn him if an attack was launched.

In the second call, after the Capitol riot, Milley told Li: “We are 100% steady. Everything’s fine. But democracy can be sloppy sometimes.”

Trump was impeached for a second time for inciting the riot, but was acquitted in a Senate trial. More than 600 people have been charged over the attack, around which five people died. This weekend supporters of those charged will rally in Washington. Capitol police, overwhelmed on 6 January, have been preparing increased security.

As Trump retains a firm grip on the Republican base and the party in Congress, a House committee containing only two dissident Republicans continues to investigate the 6 January attack.

Milley was a central player – and apparent key source – in a slew of books on Trump’s presidency that came out this summer.

In I Alone Can Fix It, by the Washington Post reporters Carol Leonnig and Philip Rucker, Milley is described before the Capitol attack resisting Trump’s demands that the military be used against anti-racism protesters while fearing a “Reichstag moment”, a coup by supporters of a president preaching “the gospel of the Fuhrer”.

Milley is also shown assuring the House speaker, Nancy Pelosi, that Trump would not be able to use nuclear weapons. Susan Glasser of the New Yorker, whose own Trump book will come out next year, reported that Milley worked to stop Trump attacking Iran.

Woodward and Costa report that Pelosi told Milley: “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?

“You know he’s crazy. He’s been crazy for a long time.”

According to Woodward and Costa, Milley said: “Madam Speaker, I agree with you on everything.”

Woodward and Costa also say the general was aware of parallels to the case of James Schlesinger, the secretary of defense who in 1974 took steps to safeguard against presidential abuse of the nuclear arsenal during the downfall of Richard Nixon – a downfall in large part caused by Woodward, in his work with Carl Bernstein investigating the Watergate scandal.

Woodward and Costa also report that concerns about Trump spread among other senior national security staff. Gina Haspel, then director of the CIA, reportedly told Milley: “We are on the way to a rightwing coup.”