Shortly before the 2020 election, Donald Trump’s national security adviser, Robert O’Brien, “stunned” the chairman of the joint chiefs of staff by saying the president wanted to kill a senior Iranian military officer operating outside the Islamic Republic.

“This was a really bad idea with very big consequences,” Mark Esper, Trump’s second and last secretary of defense, writes in his new memoir, adding that Gen Mark Milley suspected O’Brien saw the strike purely in terms of Trump’s political interests.

A Sacred Oath: Memoirs of a Defense Secretary in Extraordinary Times will be published next week. The Guardian obtained a copy.

Throughout the memoir, Esper presents himself as one of a group of aides who resisted bad or illegal ideas proposed by Trump or subordinates – such as the proposed strike on the Iranian officer.

Among other such ideas that were discussed, Esper says, were sending “missiles into Mexico to destroy the drug labs”; sending 250,000 troops to the southern border; and dipping the decapitated head of a terrorist leader in pig’s blood as a warning to other Islamist militants.

Trump made belligerence towards Tehran an important part of his administration and platform for re-election, pulling out of the Iran nuclear deal and regularly warning in bombastic terms of the cost of conflict with the US.

He also ordered a drone strike on a top Iranian general blamed for attacks on US targets. In January 2020, Qassem Suleimani, the head of the elite Quds force, was killed in Baghdad.

At a meeting in July 2020, Esper writes, O’Brien pushed for military action against Iran over its uranium enrichment – work that accelerated after Trump pulled out of the nuclear deal.

Esper’s book is subject to occasional redactions. In this case, it says “O’Brien was pushing for” one blacked out word “and military action”. Esper says the vice-president, Mike Pence, “subtly lean[ed] in behind” O’Brien, who said: “The president has an appetite to do something.”

Esper writes that Mark Meadows, Trump’s chief of staff, “jumped in to contradict this statement” and the moment passed.

However, a month or so later, on 20 August, Esper says Milley told him O’Brien had called the evening before, to say “the president wanted to strike a senior military officer who was operating outside of Iran”.

Esper writes: “Milley and I were aware of this person and the trouble he had been stirring in the region for some time. But why now? What was new? Was there an imminent threat? What about gathering the national security team to discuss this?

“Milley said he was ‘stunned’ by the call, and he sensed that ‘O’Brien put the president up to this,’ trying to create news that would help Trump’s re-election.”

Milley, Esper writes, told O’Brien he would discuss the request with Esper and others.

“I couldn’t believe it,” Esper writes. “I had seen this movie before, where White House aides meet with the president, stir him up, and then serve up one of their ‘great ideas’. But this was a really bad idea with very big consequences. How come folks in the White House didn’t see this?”

Fears that Trump might provoke war with Iran persisted throughout his presidency, stoked by reports of machinations among hawks on his staff. Such fears intensified as the 2020 election approached and Trump trailed Joe Biden in the polls.

In September 2020, Trump tweeted: ““Any attack by Iran, in any form, against the United States will be met with an attack on Iran that will be 1,000 times greater in magnitude!”

In the case of O’Brien’s suggested strike on the Iranian officer, Esper writes that he told Milley he would do nothing without a written order from Trump.

“There was no way I was going to unilaterally take such an action,” he writes, “particularly one fraught with a range of legal, diplomatic, political and military implications, not to mention that it could plunge us into war with Iran.”

He also says the O’Brien call to Milley in late August was “the last time something involving Iran seriously came up before the election”.