These ought to have been the honeymoon days of Trump-era diplomacy. With no major foreign policy crisis to troubleshoot and world leaders anxious to decipher what, exactly, America First would look like on the global stage, President Trump had an opportunity to reassure allies and start laying the foundation for joint approaches to international challenges.

Then the president began picking up the “beautiful” Oval Office phone. In the span of a couple of weeks, Mr. Trump has rattled the world by needlessly insulting allies and continuing to peddle the dumbfounding narrative that the United States has long been exploited by allies and foes alike.

His administration has not departed radically from some core positions it inherited from the Obama administration. Last week, for instance, it admonished Russia over its destabilizing role in eastern Ukraine and signaled unease about Israeli settlements. Yet Mr. Trump’s pugnacious approach to foreign relations and his first executive orders — the most misguided of which was the sweeping travel ban targeting people from seven predominantly Muslim nations — have already undermined America’s standing. The fallout has included large demonstrations in Europe, searing news coverage (the latest cover of the German newsweekly Der Spiegel features an illustration of Mr. Trump holding the severed head of the Statue of Liberty) and strong rebukes from United Nations officials.

It began, predictably, with Mexico. Mr. Trump made America’s southern neighbor and third-largest trading partner the prime punching bag of his campaign. While Mexico’s president, Enrique Peña Nieto, has met the deluge of insults and provocations with exemplary restraint, White House officials have done nothing to dial down the tension. Late last month they insisted they would find a way to bill Mexico for a border wall, perhaps by slapping taxes on imports. This message left Mr. Peña Nieto no option but to cancel a trip to Washington that had been arranged to begin undoing the damage. In a subsequent call between the two leaders, Mr. Trump reportedly threatened Mr. Peña Nieto with deploying troops across his border to take care of “bad hombres.” This was, his aides later claimed, just a joke.