LONDON — It is not normal for the United States to have two diplomatic channels for dealing with a foreign ally at war, as the U.S. apparently did with Ukraine under President Donald Trump, as the acting ambassador to Ukraine, Bill Taylor, told the House impeachment inquiry this month.

The first was the official one run by Taylor, aimed at supporting Ukraine in its war with Russian-backed separatists. The other was “irregular, informal” and unaccountable to Congress, with the goal of getting Ukraine’s new leader to do President Donald Trump “a favor” by investigating a political rival, as described by a number of witnesses — most explosively by the U.S. ambassador to the European Union, Gordon Sondland, on Wednesday.

What is also not normal is the United States’ current standing in the world and the way other countries have engaged with it since Trump took office, but particularly since the revelations about his actions toward Ukraine prompted the impeachment inquiry against him.

Diplomatic and foreign policy experts tell NBC News that the president’s habit of deviating — sometimes wildly — from long-held alliances and diplomatic norms have substantially altered America’s relations with allies around the world, and made trusting U.S. intentions and policy positions increasingly difficult.

“The U.S. traditionally has been the country that has most carefully parsed its sentences and words, and with any statement and policy it ran things through an inner-agency process where everyone is involved — what you see is what you get,” says Toomas Hendrik Ilves, who was president of Estonia from 2006 to 2016 and was foreign minister before that.

“Other countries may waffle and say ‘that’s not the case’ or ‘we didn’t mean that’ but with the States, anything from the president was always so nailed down,” Ilves said.

That’s why when something like Trump’s July 25 phone call with Ukraine President Volodymyr Zelenskiy comes up, “everyone is bewildered.”

Ilves, in a phone interview from Stanford University, where he is a visiting fellow at the Center for International Security and Cooperation, said that international actors responsible for statecraft are now wary of what the U.S. might do while Trump is in the White House.

“Most of my colleagues and people in the same position today are bewildered and trying to do their best to avoid landmines,” Ilves said. “There is an overarching and abiding concern about what will happen to the various treaty obligations that the U.S. has.”

Trump has pulled the U.S. out of several key international agreements, including the Iran nuclear deal, the Trans-Pacific Partnership trade deal and the Paris climate accords. Trump has reportedly discussed leaving NATO with senior aides and has criticized its members for not matching U.S. spending on defense — although in public he has committed to staying in the alliance.

Nevertheless, Trump’s less-than-fulsome support for NATO left Pentagon officials scrambling to reassure allies that the U.S. would meet its obligations.

“Of course renouncing treaties is more complex than simply not fulfilling a verbal promise. But I think heads of statement and governments generally are anxious and nervous,” Ilves said.

Ilves dealt with the George W. Bush and Barack Obama administrations and describes the phone calls and meetings with each president as meticulously planned.

He said that, with previous administrations, anything the president said was policy, whereas now many are left wondering what is approved and what isn’t.

Ukraine's President Volodymyr Zelenskiy listens during a bilateral meeting with President Donald Trump in New York City on Sept. 25. Jonathan Ernst / Reuters

Perhaps even more damaging in the eyes of other nations was the Trump administration’s decision to publish a transcript, albeit partial, of the Zelenskiy call, which marks a watershed moment in how other countries engage with the White House, experts said.

“What it probably does is make people a bit more cautious about what they say in those meetings and calls,” said Reza Afshar, a former senior British diplomat and now policy director at Independent Diplomat, an advisory group based in New York.

“I’m sure European states will be thinking about how they record information and are making sure they are above any impropriety,” he said. “I would imagine some states are thinking about just how they go about those discussions and making sure they are covered legally.”

Trump’s unconventional ways are seen as a strength by supporters and a detriment by critics. But in the realm of foreign affairs, where even the slightest change in language and tone can have wide-ranging effects, this unpredictability can cause problems.

“You can’t rely on what was said a couple of weeks earlier,” said Afshar, who was previously in charge of Syria policy at the U.K. Foreign Office. “In terms of the autonomous administration in northeast Syria, they were given assurances just a few weeks ago that border security would be handled by the Americans and it allowed them to pull out their own heavy weapons and personnel.”

As part of the deal agreed in August, the Syrian Democratic Forces withdrew from the Syria-Turkey border area and dismantled defenses while U.S.-Turkish forces patrolled the region. But this policy was left in tatters on Oct. 7, when Trump announced that U.S. forces would withdraw from the region, two days before Turkey’s invasion began.

“So you’re left with that assurance, and weeks later suddenly it’s meaningless,“ Afshar said.

Any suggestion of wrongdoing by the Trump administration in the Ukraine affair, however strongly denied, could also have ramifications for how other states interact with the U.S., its allies and rivals, he added.

The shift doesn't "change what Russia and China do, they do these things anyway, but it gives the impression that what they are doing is the norm and gives them encouragement,” Afshar said.

In several ways, it was unfortunate that Ukraine became embroiled in an international scandal with the U.S.

“They didn’t only have a strong bilateral relationship, Ukraine viewed the United States as Not Russia,” said Jennifer Cassidy of Oxford University, a former diplomat with the Irish government, European Union and the United Nations

“If you look at it from a Ukrainian point of view, when they looked at the transcripts and what was said, it turned the U.S. from a model of good governance and truth into what they are trying to get away from, the sort of corruption they are battling.”

Cassidy teaches her politics students at Oxford about what she calls the “shadow of the past” — the idea that whenever two states interact they should always do so on the assumption that they will have to do business together again, so you should always keep relations positive and ongoing.

Rarely do states deviate from this way of working — but the current U.S. administration is not known for doing things by the book.

“This is historically how diplomats and heads of state have always negotiated — they would never cut off all diplomatic ties, or that would happen very rarely,” Cassidy said.

“What we’ve seen from the Trump administration is that it’s just a short-term gain. There is no regard for how this is going to impact America’s reputation, its credibility and its trust on the international stage.”

The most important effect of this short-termism, Cassidy argues, is how the U.S.’s traditional foes might react in the future.

The Iran nuclear deal that Trump abandoned was a complex multilateral treaty signed not just by Iran and the U.S. but Russia, China, Germany, France and the United Kingdom, which have spent the last few months attempting in vain to resurrect some form of the agreement on their own.

“Whether you agree with Iran or not, you can still see the logic of the view that they now hold the card of credibility. Why would they ever come back to the negotiating table with a country that’s wasted their time?” Cassidy said.

“Countries hostile to America hold the cards to not negotiate with America. This lack of long-term strategy, especially if it’s for eight years, is going to be severely damaging to the U.S.’s reputation.”