The same day, when Trump tweeted for the umpteenth time that Mexico “is not doing nearly enough” to apprehend drug smugglers and migrants, and threatened to send “ARMED SOLDIERS” to the border, Mexican President Andrés Manuel López Obrador figuratively shrugged.

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Mexico would, of course, look into any U.S. concerns, López Obrador said at a Mexico City news conference. But “the most important thing is to tell [Trump] that we are not going to fight with the government of the United States. . . . We are not going to be tripped up by any provocation.”

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Friendly countries that have been the target of Trump’s tweets have learned he is not always bluffing. Aluminum and steel tariffs really were imposed on allies, and the Pentagon is sending more troops to assist in Border Patrol operations.

But few now take Trump’s Twitter fulminations at immediate face value. “He has shown us that what’s black at 9 a.m. can be gray at 3 p.m. and white at 7 p.m.,” said a Mexican diplomat of the president’s revolving pronouncements.

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Most have learned that responding in kind only makes matters worse. “If you don’t pick a fight with him, you take away from Trump the possibility of further enraging himself and everybody else,” said the diplomat, who spoke on the condition of anonymity to avoid fueling any fires.

Gérard Araud, France’s U.S. ambassador until earlier this month, said he told his government early on that tit-for-tat tweets were counterproductive.

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First, Araud said, Trump’s tweets “are largely for domestic consumption. . . . Second, it’s totally useless to answer, because basically it will lead to escalation, and, to be frank in terms of vocabulary, President Trump has an escalation dominance.”

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In December, Trump indirectly hammered French President Emmanuel Macron, saying that widespread demonstrations across France were in response to the cost of the 2015 Paris climate agreement from which Trump withdrew. The protesters, Trump tweeted, inaccurately, were chanting, “We Want Trump!”

The response was left to Foreign Minister Jean-Yves Le Drian, who noted dryly in a television interview that demonstrators “were not protesting in English, as far as I know.”

Macron, on the advice of aides, “didn’t engage in childish teenaged behavior,” Araud said, so his government “avoided the worst.”

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“You can talk to my German or my British colleagues,” he said. “Every European leader has been the target at some moment of tweeting outbursts. That’s part of this strange period that we are living in.”

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Trump supporters and some White House officials, however, say that foreign leaders ignore Trump’s outbursts at their peril. Aides said that the president’s tweets are aimed at his domestic audience and meant to give him leverage. Sometimes, he will poll those around him and ask how a particular line of attack might go over with a foreign leader, sizing up their strengths and weaknesses, current and former officials said.

“If they aren’t taking him seriously, that’s a mistake,” said Tom Bossert, former White House homeland security adviser. “The president’s style is an interpersonal negotiation style, whereas we’ve seen other presidents take an institutional style. Even his harsh language is an outgrowth of his interpersonal negotiation style.”

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Barry Bennett, a Trump ally and lobbyist for foreign governments, said that, while others may be “getting used to his behavior, they don’t doubt he would act against them.”

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“He’s proven that he’s not usually just bluffing,” Bennett said. “The Germans took the [threat of] car tariffs very seriously. . . . I don’t remember the Germans flying over here to talk about zero tariffs on cars during the Obama administration.”

Trump had been insulting Kim Jong Un for years on Twitter before he warned, as president, that he would rain down “fire and fury” on the North Korean leader. It was the only message the dictator understood, Trump told senior aides gathered in the Oval Office. “Man versus man, me versus Kim,” Trump said, according to several aides who spoke on the condition of anonymity to discuss internal deliberations.

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When Kim responded in kind, Trump tweeted back asking why the North Korean would “insult me by calling me ‘old,’ when I would NEVER call him ‘short and fat?’ ”

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In Kim’s case, trading barbs apparently worked, at least temporarily, leading to two summits and a pause in North Korean nuclear tests and missile launches.

There has been no similar result with Iran, an unending target of threatening tweets, many in all-caps for emphasis. Supreme Leader Ayatollah Ali Khamenei and President Hassan Rouhani generally respond with serious-sounding counterpunches, while Foreign Minister Mohammad Javad Zarif often strives for wit.

When Trump segued in a November statement on the killing of Saudi journalist Jamal Khashoggi to a litany of complaints about Iran, Zarif called the statement “bizarre” and noted that “perhaps we’re also responsible for the California fires because we didn’t help rake the forests — just like the Finns do?” a take on Trump’s musing that Finland had avoided such conflagrations by clearing their forest floors.

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Ostensible friends, at least at the beginning, were often the most unnerved by the tweets and frequently turned to those around Trump for explanation or reassurance.

For Germany, whose Chancellor Angela Merkel has been a frequent target, former defense secretary Jim Mattis was the go-to explainer, although German officials said they usually would find that Mattis agreed with them rather than Trump.

British Embassy officials have invited a raft of officials to the ambassador’s residence for dinner — including former chief of staff John F. Kelly, national security adviser John Bolton and advisers Stephen Miller and Kellyanne Conway — to help them understand Trump’s more puzzling pronouncements.

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Kelly, before resigning in December, told military and other officials that Trump’s tweets should not be seen as official government policy and they should wait to see it on paper.

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Mexico has long looked to White House adviser and Trump son-in-law Jared Kushner as an ally, as well as Cabinet members such as Agriculture Secretary Sonny Perdue and U.S. business leaders, when the president channeled his ire over migrants toward Mexico and threatened to impose new tariffs or close the border.

Mexican officials are among the most inured to being in the Twitter crosshairs — charged as they were during the campaign with sending criminals and “rapists” into this country.

“There were a lot of expectations that he was going to be a different political animal once he was in office,” the Mexican diplomat said. “And he wasn’t.” Little has changed over the past two years, but “the surprise element is gone.”