U.S. President Donald Trump risks diminishing his country’s international standing if his hostile behaviour towards world leaders continues, but the harm won’t be permanent, says an expert in global affairs.

“The good news is that America’s alliances are very deeply institutionalized. They have a long historical tradition and they’re very resilient,” said Hal Brands, who holds the Henry A. Kissinger distinguished professor of global affairs at Johns Hopkins University School of Advanced International Studies.

“The problem is that American allies cannot help but doubt American reliability and credibility at a time when they are frequently being abused or lambasted by the president in the press or in private. And one fears that this might also have an emboldening effect on American adversaries by making them think that perhaps U.S. alliance guarantees aren’t as iron-clad as they once were.”

Reports of tense phone calls this week with Mexican President Enrique Pena Nieto and Australian Prime Minister Malcolm Turnbull, calls that are traditionally amicable chats between a new president and an allied leader, have sparked new fears about Trump’s ability to stickhandle international relations.

(Officials in both the White House and Mexico's administration disputed reports that the call included a threat to send U.S. troops to Mexico. Turnbull said Friday that he and Trump had a "frank discussion" that ended "courteously," which contradicts news reports that said Trump had hung up on Turnbull.)

Brands says it’s not unprecedented for American presidents to have difficult relationships with allied leaders. In fact, Richard Nixon spoke in crass and confrontational terms with foreign leaders and Lyndon Johnson had a “famously testy relationship” with former Canadian prime minister Lester B. Pearson, he says.

“What’s different in this case is that President Trump has now done this with a series of U.S. allies and partners over the past couple of weeks,” he told CTV’s Your Morning on Friday. And he’s seemingly proud of this brash persona he’s created, while presidents typically pride themselves on being presidential.

But it goes deeper than words. Brands says Trump doesn’t view allies as any more special than other countries, and that his view is “immoral transnationalism” in that he is just as willing to cut a deal with China or Russia as a traditional ally. He’s called NATO “obsolete,” mused about lifting sanctions on Russia and repeatedly threatened to walk away from trade deals if they aren’t renegotiated to his liking.

“It seems to be reflective in Trump’s case of a long-standing view that American allies and partners are basically freeloaders who are fattening themselves at the American trough. This is something that he’s repeatedly come back to since the 1980s. So I think the consistency and the level of abuse to which he has subjected American allies and partners is something quite new.”

It’s also having an effect.

U.K. Prime Minister Theresa May is under fire for what critics called her “normalizing” of Trump’s policies during her state visit last week. A petition demanding an invitation for the president to visit the U.K. be withdrawn, which has been signed by more than 1.8 million people, will be debated in parliament Feb. 20.

Brands says alliances with Canada and the U.K. will endure because they are bigger than any one president but leaders of both countries should worry about a unprecedented level of “unpredictability” and “hostility” from the White House.

“If President Trump decides that Canada or the United Kingdom has done something that offends him, he may very well lash out publicly as he has repeatedly done before. I think the alliances will survive over the long term but we’re going to be in for rough sledding.”

Beyond his personal relationships with foreign politicians, Brands says Trump is undermining the work of American diplomats and military personnel, who will have to reassure counterparts abroad that America remains committed to its international partnerships.

“The problem is that they will find themselves continually undercut by tweets coming from the White House, by intemperate statements coming from the president.”

Trump is also facing deep division in the department charged with overseeing foreign relations.

More than 1,000 State Department employees have expressed opposition to Trump’s highly controversial immigration ban on residents of seven Muslim-dominated countries. That follows the resignation of seven senior leaders a few days after Trump’s inauguration.

Rex Tillerson, former chief executive of ExxonMobil, was confirmed as Secretary of State Wednesday. The 43 Senate votes opposing his appointment was the most in U.S. history for that position.

"You understand that the job of our diplomats and the mission of the State Department is to serve the interests of the United States of America to make our nation safer, our country more prosperous and our country much more secure," the president told Tillerson during his swearing-in.

"I do believe we can achieve peace and stability in these very, very troubled times."

-With files from The Associated Press