PRESIDENT TRUMP has blown hot and cold about various foreign leaders. Kim Jong Un of North Korea, for example, has mutated from “Little Rocket Man” to someone with whom the president has “great chemistry.” Yet there are two figures about whom Mr. Trump has been fairly consistent: President Vladi­mir Putin of Russia is the recipient of Mr. Trump’s warmth and understanding, and Chancellor Angela Merkel of Germany is the target of his criticism and contempt. No tendency of Mr. Trump better demonstrates his underlying ideology and intentions than his disparate treatment of Moscow’s reactionary despot and Berlin’s liberal democrat.

Now Mr. Trump may be going beyond the equivalent of heckling Ms. Merkel to more direct and even less appropriate intervention in the internal politics of her country. Ms. Merkel has recently been involved in a policy and power struggle over migration with her interior minister, who doubles as the head of a Bavarian conservative party belonging to her governing coalition. The interior minister favors turning away certain asylum seekers; Ms. Merkel objects that there needs to be a unified European approach. Her government risked falling over the issue, until the two agreed to a two-week cease-fire, at which point Mr. Trump weighed in with a tweet: “The people of Germany are turning against their leadership as migration is rocking the already tenuous Berlin coalition. Crime in Germany is way up. Big mistake made all over Europe in allowing millions of people in who have so strongly and violently changed their culture!”

Forget the factual errors in these claims, including Ms. Merkel’s continuing popularity among half of Germans, according to polls. It is extraordinary for an American president to egg on the internal adversaries of a fellow NATO member, and it can only convince Ms. Merkel and other Western leaders that the U.S. presidency has been captured by someone intent on dealing with them in bad faith. Equally troubling, Mr. Trump is simultaneously reaching out to the populist right in Europe. He held a phone call with Prime Minister Viktor Orban of Hungary, in which both “agreed on the need for strong national borders,” as an official White House summary put it; the past two U.S. presidents had shunned Mr. Orban due to his illiberal governance and coziness with Moscow. Mr. Trump’s other new best friend in Europe is Italy, whose freshly installed right-left populist coalition voiced support for Mr. Trump’s call to re-admit Russia to the Group of Seven and has begun turning away boats filled with desperate migrants from Africa.

“Nowhere is it written in stone that the transatlantic bond will always thrive,” NATO Secretary General Jens Stoltenberg warned Tuesday . The 70-year-old transatlantic alliance, with all the political and economic benefits it has provided, needs constant maintenance. If it’s cared for in good faith, Mr. Stoltenberg’s warning won’t materialize. At the moment, however, what the West is getting from the president of its most powerful member is constant disparagement; worse, he’s not angry at other leaders because they aren’t upholding the alliance’s core values and purposes, but because they are.

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