At an Oval Office meeting with the then Ukrainian president, Petro Poroshenko, in 2017, Donald Trump asked his national security adviser if US troops were in Donbas, territory claimed by Russian-backed separatists, which Vladimir Putin last month used as pretext for a full and bloody invasion.

Describing the meeting in a new book, the then US ambassador to Ukraine, Marie Yovanovitch, writes: “An affirmative answer to that question would have meant that the United States was in a shooting war with Russia.”

Yovanovitch adds: “I pondered whether it was better to interpret Trump’s question as suggesting that the commander-in-chief thought it possible that US troops were fighting Russia-led forces, or instead as an indicator that the president wasn’t clear which country was on the other side of the war against Ukraine.

“Either way, it was disconcerting that he did not seem to know where we had our troops – his troops – deployed. I could only imagine what the Ukrainians were thinking.”

Trump fired Yovanovitch in 2019, amid attempts to withhold military aid to Ukraine in return for political dirt on Joe Biden and other rivals, an affair which fueled Trump’s first impeachment.

Yovanovitch describes the Trump-Poroshenko meeting in Lessons from the Edge: A Memoir, which will be published on Tuesday. The Guardian obtained a copy.

The book comes three weeks into the Russian invasion of Ukraine, which followed years of proxy warfare in the east of the country.

Yovanovitch also writes that Trump told Poroshenko Ukraine “was a corrupt country, which he knew because a Ukrainian friend at Mar-a-Lago had told him”.

Trump, she says, also said: “Crimea was Russian, as the locals spoke Russian”.

Russia annexed Crimea in 2014, a move never recognized by the international community. Yovanovitch writes that Trump’s words were “surprising enough to hear from one head of state to another” but Trump topped them by asking his national security adviser, HR McMaster, whether US troops were in Donbas.

“Everyone kept a poker face on,” she writes.

Echoing descriptions of Trump’s favored working techniques by multiple close aides, Yovanovitch says Poroshenko deployed “visual aids, which Trump really liked” as he “ably pushed back” and made his case for support.

Poroshenko requested the inclusion of Javelin anti-tank missiles in a package of security aid. Trump seemed open to the idea, Yovanovitch writes. In 2019, however, news broke of his attempt to withhold military aid and secure dirt on Biden.

Yovanovitch’s book comes as Poroshenko’s successor, Volodymyr Zelenskiy, leads his country’s fight against Russian invaders, his forces using US-supplied Javelins and other weapons sent by allies.

The Poroshenko meeting was brief and forms a small part of a book which tells Yovanovitch’s story of machinations involving Rudy Giuliani, Trump’s attorney, which led to her firing and Trump’s impeachment.

But her description of the meeting echoes others by sources including John Bolton, McMaster’s successor as national security adviser, which have shown Trump risking embarrassment and mishap when one-on-one with world leaders.

Trump’s ambassador to Israel, David Friedman, recently revealed that Trump risked disaster in an early meeting with his counterpart Reuven Rivlin, when he praised the Palestinian leader Mahmoud Abbas and criticized Benjamin Netanyahu, then the Israeli prime minister, for being unwilling to seek peace.

Trump’s comments “knocked everyone off their chairs”, Friedman wrote.

Participants in the meeting with Poroshenko appear to have stayed seated.

Yovanovitch writes that she sensed “Trump had come into the meeting viewing Ukraine as a ‘loser’ country, smaller and weaker than Russia”, only to be “a little surprised by Poroshenko”, who was “as physically imposing as Trump” and who was also “a billionaire businessman”.

After the meeting, Trump said Ukraine was “a place that everybody’s been reading about”. Poroshenko told reporters he was “satisfied with the results of the negotiations”, and said the two leaders discussed military and technical cooperation.

Yovanovitch “hoped that Poroshenko had created the kind of favorable impression that would make Trump rethink his views of Ukraine and its importance to our strategic interests”.

However, she adds, “Trump’s obsequiousness toward Putin was a frequent and continuing cause for concern”.

In 2018, Trump staged an infamous summit with Putin in Helsinki at which the two men spoke in private for close to two hours. Trump’s “toadying up” to Putin at the press conference which followed, Yovanovitch writes, made her lose her appetite.

“When the Ukrainian media called,” she writes, “… we took the opportunity to reinforce the point that US policy was to help Ukraine defend itself against Russian aggression”.

Five years on from Trump’s meeting with Poroshenko, with Ukraine in a fight for its existence, Trump seems not entirely to have shed his suspicion that US troops could be in the country – a step the Biden administration has made clear will not be taken, given the potentially huge cost of confrontation with Russia.

Last month, Trump appeared to misunderstand a Fox News host, to the extent of believing Americans troops had landed in Ukraine.

“You shouldn’t be saying that, because you and everybody else shouldn’t know about it,” the former president said, seemingly mistaking reports of Russian troop movements for US ones. “They should do that secretly, not be doing that through the great Laura Ingraham.”

“No, those are the Russians,” Ingraham corrected him.

“Oh, I thought you said that we were sending people in,” Trump said. “That’ll be next.”