A week before election day 2020, the US secretary of defense was so worried that Donald Trump would seek to involve the military in the election in an attempt to hold on to power, he told the general commanding the US national guard to notify him of any communication from anyone at all at the White House.

“Without being too explicit,” Mark Esper writes in a new memoir, “my message was clear: the US military was not going to get involved in the election, no matter who directed it. I would intercede.”

Such an intercession, Esper writes, would involve trying to persuade Trump not to use the military to hold on to power, then if necessary Esper would resign, appeal to Republicans in Congress and hold a press conference to appeal to the American people directly.

Esper thought Trump might order actions such as seizing ballot boxes in key states. Ultimately, Trump did not attempt to use the military to influence the election, which he lost to Joe Biden. He did seek to overturn the result by other means.

Esper was fired by Trump on 9 November 2020, six days after the election.

He details the extraordinary steps he felt compelled to take before that in a new book, A Sacred Oath: Memoirs of a Defense Secretary in Extraordinary Times, which will be published next week. The Guardian obtained a copy.

Esper devotes considerable space to his work with Gen Mark Milley, the chairman of the joint chiefs of staff, to stymie Trump’s attempts to use the military for political purposes, either in military strikes against Iran or in Syria – or even Mexico – or at home, by invoking the Insurrection Act against protesters for racial justice.

Trump’s request that such protesters be shot in the legs, and Esper’s account of his resistance to it, has been reported elsewhere.

The protests died down but Esper says the two most senior Pentagon figures remained concerned Trump could seek to use the military domestically, to tilt power his way.

Esper describes a meeting at the Pentagon with Milley and the national guard chief, Gen Daniel Hokanson, on 30 October, “the last Friday before the election”.

The “ostensible purpose of the meeting”, Esper says, given it was visible to anyone who could see his calendar, was an update on national guard military police units placed on alert to placate Trump during the protests for racial justice.

But with regard to the election, Esper says, “this was a serious moment”.

He says he told Gen Hokanson: “If at any point in the coming days – before, during or after the election – you get a call from anyone at the White House, take it, acknowledge the message, and call me immediately. The same rule applies if you hear of any TAGS [national guard state adjutant generals] or governors getting a similar call.”

Esper says he also asked Hokanson “to figure out a discreet way to get this last part out, which he said he would”.

Esper writes that as the only civilian between the president and the military, he was concerned the White House might “try to circumvent me to do something inappropriate” and “wanted to be ready for anything”.

“The whole point of my game plan – the reason that I had taken so much crap over the last several months – was to be in this position, at this moment, to act. The essence of democracy was free and fair elections, followed by the peaceful transition of power.”

Milley, he writes, told him he and the other joint chiefs would “resign if pressed to break their oath” and involve the military in the election. Esper says he did not want to allow the generals “to be put in such a compromising position, especially if a presidential order was legal but grossly wrong or inappropriate.

“… If such an order came from the White House, my immediate recourse would be to demand a meeting with the president. I would want to hear and understand the directive straight from him, to offer alternative solutions if such were possible, and to voice my opposition face-to-face if he was unyielding. If I was unsuccessful I would be forced to resign on the spot in protest. But that wouldn’t be the end of the line for me.”

Esper writes that he would have called senior Republicans on Capitol Hill to ask them to intercede with Trump, then staged a press conference “where I informed the country about all that had transpired and continued to unfold.

“I would present my best case and make an appeal to the American people, their elected leaders in Congress, and the institutions of government to intervene. The point would be to buy time and put pressure on the president to stand down.”

As Esper writes, election day, 3 November, “came and went without any incident involving the armed forces. Thank God.” He says he was relieved that though the contest was not called until the following weekend, Biden’s lead was clear.

Esper also says he “never imagined” what came next: Trump’s attempt to overturn the election through lies about electoral fraud and coordination with Republicans in Congress and other rightwing groups and advisers, culminating on 6 January 2021 in the storming of the US Capitol by a pro-Trump mob.

The attack on the Capitol has been linked to seven deaths and has led to more than 800 charges. But it failed to stop certification of Biden’s win. Having been fired after the election, Esper watched the attack on TV.