President Trump's political rhetoric on North Korea has differed from before he declared his candidacy to now. (Taylor Turner/The Washington Post)

In a string of tweets fired off Sunday morning from Hanoi, Vietnam, President Trump responded with sarcastic insults to a recent message from the North Korean government that had referred to him as “old.”

“Why would Kim Jong-un insult me by calling me 'old,' when I would NEVER call him 'short and fat?' " Trump wrote in his tweet, referring to the leader of North Korea's ruling dynasty. “Oh well, I try so hard to be his friend — and maybe someday that will happen!”

The message marks an unusually personal escalation of the tensions between the United States and North Korea over Pyongyang's weapons program. It is also another sign of the change in rhetoric used to address North Korea since Trump took office: Though North Korea has long been known for hurling bellicose insults at world leaders, rarely have those world leaders responded in kind.

Of course, Trump is a not your average world leader. The current president is a pugnacious social-media user often willing to respond with his own harsh words when he feels wronged. As a spokeswoman for his wife, Melania Trump, put it earlier this year, when Trump is attacked, “he will punch back 10 times harder.”

Whether this instinct to hit back could help his self-described efforts toward becoming Kim's friend in the future — or harm them — is unclear.

President Trump harshly criticized North Korean leader Kim Jong Un at the United Nations on Sept. 19, calling him "Rocket Man" and threatening to "totally destroy North Korea" if need be. (The Washington Post)

The North Korean message that aggrieved Trump was released Saturday by the country's Foreign Ministry and described Trump's 12-day tour of Asia as “a warmonger's trip for confrontation with our country, trying to remove our self-defensive nuclear deterrent.'' The statement also said the “reckless remarks by an old lunatic like Trump will never scare us or stop our advance.”

The North Korean government has insulted Trump personally numerous times. Its state-run media has run a number of unflattering descriptions of Trump, including the memorable use of the word “dotard” in September. It has frequently referred to Trump as “old” and accused him of being a “war maniac” and a “lunatic.”

These insults come at a time of heightened tension between Washington and Pyongyang. North Korea has pushed ahead with its weapons program over the past few months, conducting a number of long-range missile tests, plus a nuclear bomb test, since Trump took office.

However, the insults also fit into a long tradition of insulting American leaders. In 2014, the U.S. government criticized a lengthy racist screed published by North Korea's State-run Korean Central News Agency that had referred to President Barack Obama as a “dirty fellow,” among other things.

In recent years, North Korea has also insulted former secretaries of state John F. Kerry (“hideous lantern jaw”) and Hillary Clinton (both a “schoolgirl” and a “pensioner"), while the entire administration of President George W. Bush was referred to as “a bunch of tricksters and political imbeciles.” The Americans have not responded with their own public insults, though Bush did privately call Kim's father, Kim Jong Il, a “pygmy” in 2002, according to reports at the time.

Trump's descriptions of North Korea's current leader have varied, and he has even been positive at times —  describing him as a “pretty smart cookie” in April. But as tensions with North Korea have escalated, so too has the harshness of the American president's rhetoric, with Trump dismissively referring to Kim as “Little Rocket Man” and warning of “fire and fury” if North Korean threats continued — a statement that perhaps inadvertently echoed North Korean propaganda.

Some had worried that Trump would use similarly personal and angry language while in South Korea last week and run the risk of inciting the North. However, though his speech to South Korea's National Assembly was deeply critical of North Korea, it was less bombastic and more measured than his previous statements.

That speech was drawn up carefully with the input of others in Trump's administration. Trump, however, is a famously impulsive tweeter.

Worse still for both sides, the insults may hit sensitive spots. Trump is the oldest first-term president in U.S. history and more than twice the age of the North Korean leader. Meanwhile, Kim's height is estimated to be 5-foot-7, and he is rumored to suffer health problems due to his weight.

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