SYDNEY — For decades, Australia and the U.S. have enjoyed the coziest of relationships, collaborating on everything from military and intelligence to diplomacy and trade.

Yet an irritable tweet President Donald Trump fired off about Australia and a dramatic report of an angry phone call between the nations’ leaders proves that the new U.S. commander in chief has changed the playing field for even America’s staunchest allies.

Prime Minister Malcolm Turnbull was left scrambling to defend his country’s allegiance to the U.S. after The Washington Post published a report on Thursday detailing a tense exchange that allegedly took place during the Australian leader’s first telephone call with Trump since he became president.

During the call, the Post reported, Trump ranted about an agreement struck with the Obama administration that would allow a group of mostly Muslim refugees rejected by Australia to be resettled in the United States. The newspaper said Trump dubbed it “the worst deal ever” and accused Turnbull of seeking to export the “next Boston bombers” — a reference to Tamerlan and Dzhokhar Tsarnaev, U.S. citizens born in Kyrgyzstan who set off explosives at the 2013 Boston Marathon.

Though Turnbull declined to confirm the report, he also didn’t deny it, apart from rejecting one detail — that Trump had hung up on him. The prime minister insisted his country’s relationship with the U.S. remained strong, and that the refugee deal with the U.S. was still on.

Yet shortly after, Trump took to Twitter to slam the agreement, tweeting: “Do you believe it? The Obama Administration agreed to take thousands of illegal immigrants from Australia. Why? I will study this dumb deal!”

Australians, long accustomed to a chummy friendship with the U.S., were stunned by the drama. Not since the Vietnam War — when Australia’s then-Prime Minister Gough Whitlam criticized a series of bombings authorized by then-President Richard Nixon — has there been such obvious friction.

“You can’t help but think the signal this sends to world leaders: That you have to be very, very careful doing business with this administration, particularly with the president and the people around him,” said Simon Jackman, CEO of the U.S. Studies Center in Sydney. “And that can’t help but put a chill on relations between allies.”

Yet the only surprising thing about Trump’s reaction to the deal is that Australians were surprised at all, said Nick Economou, a political analyst at Monash University in Melbourne.

“I suspect that there is a feeling that, ‘Oh no, we’ve dealt with Republicans before, we were very close to George W. Bush, we should be fine with Mr. Trump and he’ll agree to this deal,”’ Economou said. “But the thing is, of course he’s not going to agree to this deal! Obama entered into it and whatever Obama was for, Donald Trump is against.”

Australia has long been one of America’s strongest allies. The nation has fought alongside the U.S. in every major conflict since World War I, including the Korean War, Vietnam War and, more recently, in the Middle East. Australia is also part of the “Five Eyes” intelligence-sharing program with the U.S., along with Canada, Britain and New Zealand.

And while few believe the spat over the refugee deal will permanently damage those ties, it will likely prompt changes in how America’s allies approach their dealings with Washington.