Europeans are now finally being forced to realize that Mr. Trump’s world is one shorn of allies.

Europe’s reaction to this new reality brings to mind Stevie, the heroine from Edward Albee’s play “The Goat, or Who Is Sylvia?” when she learns that her husband has fallen in love with a goat. The shock is so great that she simply refuses to understand. That’s basically what Europe is doing right now. In the lead-up to the NATO summit, presidents and prime ministers reiterated familiar lines about trans-Atlantic relations as though nothing had changed with Mr. Trump’s election.

Here’s why the European leaders are so surprised: It’s not difficult to imagine a mainstream political leader campaigning with promises to build a wall on the border, move the American embassy in Israel to Jerusalem and punish the Chinese government for its economic policies. Mainstream politicians might easily pledge all of these, but upon assuming power they would never act on them.

Chancellor Angela Merkel of Germany and President Emmanuel Macron of France may have had a similar mind-set about Mr. Trump. They read his tweets and waited for him to show up at a Group of 7 meeting, sure that he would never follow through on proposals. How could he, in the face of all the adults in the room? And yet, he has. Again and again.

Like today’s other populist leaders, Mr. Trump knows that his standing with voters hinges on making good on his most radical promises. For a populist leader to succeed, he or she doesn’t need to solve problems, nor outdo his or her predecessor. All the populist leader has to do is be different from the mainstream — to do what mainstream politicians would never do. For example, insult Germany.

What makes Europeans particularly vulnerable in the Trump era is that they view themselves as America’s allies. (That’s for good reason: They have been, after all, America’s allies for the past 70 years.) But in Mr. Trump’s world, there no longer is any concept of alliances. It is not that he is displeased with European military spending or with Europe’s position on Iran. Rather, it is that in a world where America is a disrupter and not a force for stability, allies are now a burden. They have expectations and claims that constrain America’s policies, whether that is a preferential trade agreement or a commitment to joint military exercises. Worse, they insist on predictability and reciprocity, which are completely out of sync with Mr. Trump’s view of the world.