INTERNATIONAL REACTION to Donald Trump’s electoral victory was as dispiriting as it was predictable. Stalwart allies of the United States in Europe and Asia reeled with shock and aversion, even as populists and dictators cheered and U.S. adversaries gloated. To judge from foreign assessments, the United States is about to place itself at the head of a global, populist, quasi-authoritarian movement that will have better relations with Russia than with liberal leaders of the European Union.

We hope that turns out to be unfounded. Mr. Trump has offered some early reassurance in phone calls to key U.S. allies: He reportedly reaffirmed the U.S. defense commitments to Japan and South Korea, and, according to Downing Street, he told British Prime Minister Theresa May that the “special relationship” between Washington and London “would go from strength to strength.” Mr. Trump’s pledge to South Korean President Park Geun-hye to be “steadfast and strong” in protecting against the threat from North Korea, as reported by Ms. Park’s office, was particularly constructive, given Mr. Trump’s campaign rhetoric about withdrawing U.S. support and encouraging South Korea and Japan to develop their own nuclear weapons.

In light of this outreach, some of the initial reactions from U.S. allies in Europe looked impolitic. Germany’s deputy chancellor, Sigmar Gabriel, declared that “Trump is the trailblazer of a new authoritarian and chauvinist international movement. . . . They want a rollback to the bad old times in which women belonged by the stove or in bed, gays in jail and unions at best at the side table.” Mr. Gabriel was referring in part to populist nationalists in Europe, including in Germany, who hope the U.S. election will lend momentum to their movements. Yet such rhetoric could only make such a political alignment more likely, by souring Mr. Trump on mainstream European leaders such as Mr. Gabriel.

That’s not to say that U.S. allies should seek to excuse or appease Mr. Trump; principled engagement with this U.S. president will be essential. That’s why we were heartened to see the statement by Mr. Gabriel’s superior, Chancellor Angela Merkel, which reaffirmed that “partnership with the United States is and will remain a keystone of German foreign policy” — and then carefully set out some conditions.

“Germany and America are bound by common values — democracy, freedom, as well as respect for the rule of law and the dignity of each and every person, regardless of their origin, skin color, creed, gender, sexual orientation, or political views,” Ms. Merkel’s statement said. “It is based on these values that I wish to offer close cooperation.”

There is encouragement in that for Mr. Trump to build a partnership with Europe’s strongest nation, in spite of some sparring between him and Ms. Merkel during the campaign. But there is also an implicit warning that Germany’s leader will call Mr. Trump out if he violates liberal democratic norms. It would be well if the president-elect heard that respectful but firm message from more of the West’s leaders as he prepares to take office. The United States has long worked to press other countries to respect human rights. Its closest friends should now be prepared, if necessary, to give it a dose of its own medicine.