President Donald Trump in New Jersey. REUTERS/Jonathan Ernst

President Donald Trump's remarkable escalation of rhetoric against North Korea this week may have been an unintentional consequence of the president's use of one of his favorite rhetorical crutches.

"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen," Trump told reporters at his golf club in Bedminster, New Jersey, on Tuesday. He added that North Korean leader Kim Jong Un had "been very threatening beyond a normal state, and as I said they will be met with fire, fury, and frankly power the likes of which this world has never seen before."

The surprisingly aggressive statement alarmed defense experts, who homed in on one turn of phrase in particular: "the likes of which this world has never seen before." Its inclusion turned what could have been an ordinary, albeit vague, warning to North Korea into one that seemed to threaten a full-scale nuclear attack.

The phrase, however, is one of Trump's most familiar rhetorical crutches, one that he uses in comments both scripted and improvised.

Trump at a rally on June 21 in Cedar Rapids, Iowa. Associated Press/Charlie Neibergall

The phrase showed up twice earlier on Tuesday, leading The Toronto Star's Daniel Dale to question whether Trump intended to use the particular language or whether it had simply been in his head from before. The first appearance was in a scripted statement about the opioid crisis, which the president called "a problem the likes of which we have not seen."

Later, speaking freely, Trump used the phrase again while emphasizing the White House's efforts to stop drugs from crossing over from Mexico.

Trump during an interview with Reuters in the Oval Office on April 27. REUTERS/Carlos Barria

"We're being very, very strong on our southern border — and I would say the likes of which this country certainly has never seen that kind of strength," Trump said.

But Tuesday was far from the only time has used the expression.

He used it in his "American carnage" speech on Inauguration Day, when he told his supporters, "You came by the tens of millions to become part of a historic movement the likes of which the world has never seen before."

He used similar language at a postelection campaign-style rally in Wisconsin, calling his supporters "a grassroots movement the likes of which the world has never seen before."

A day before his inauguration, Trump touted his picks for the incoming Cabinet — "the likes of which have never been appointed."

In a speech pledging to reform the Veterans Affairs Department in April, Trump said, "We have a team the likes of which have never ever been assembled."

In a New York Times interview in March 2016, Trump said China would build a fortress in the South China Sea "the likes of which perhaps the world has not seen." Later in the interview, when discussing China's reported gross domestic product, Trump used the phrase again.

"We'd be in a boom, the likes of which we've rarely seen before, right?" he said.

Trump used the idiom yet again after being named Time magazine's Person of the Year in 2016.

"What amazes a lot of people is that I'm sitting in an apartment the likes of which nobody's ever seen," Trump said. "And yet I represent the workers of the world."

The New York Times reported that Wednesday's "fire and fury" threat was improvised and that the president's language had not been run by his staff. But Trump reportedly used similar language when discussing North Korea in private.

"The words were his own," the White House press secretary, Sarah Huckabee Sanders, told The Times. "The tone and strength of the message were discussed beforehand."