CONGRESSIONAL TESTIMONY by Donald Trump’s national security nominees last week suggested that the incoming administration would not seek to dismantle the alliances that have undergirded the West — and U.S. global leadership — since 1945. “If we did not have NATO today, we would need to create it,” said defense secretary nominee James N. Mattis. The U.S. treaty commitment to defend its European allies, said secretary of state nominee Rex Tillerson, is “inviolable.”

It is still not clear, however, that Mr. Trump agrees. In an interview published by the Times of London on Monday, he recalled his claim last year that NATO was “obsolete” because it did not fight terrorism (though it does) and because many of its members did not meet its defense spending guidelines. He then went on to say that “it doesn’t matter” to him whether the European Union exists, predicted more countries will leave it and placed German Chancellor Angela Merkel on par with Russian President Vladi­mir Putin in meriting his trust. Unsurprisingly, Moscow hailed Mr. Trump’s words, while senior European leaders reacted with alarm.

Ms. Merkel played down the statements, and maybe she’s right: Perhaps Mr. Trump’s words — he also said “NATO is very important to me” — were haphazard and should not be taken seriously. Yet if the president-elect’s intention was to undermine the transatlantic alliance, encourage the disintegration of the European Union and tear down Ms. Merkel as she begins a reelection campaign — an agenda identical to Mr. Putin’s — he could hardly have been more effective.

Russia has already launched a disinformation campaign to discredit Ms. Merkel, using fake-news websites and Internet bots in the same way it targeted Hillary Clinton. Germans might be forgiven for thinking that Mr. Trump’s intervention was designed to enhance that assault. He accused Ms. Merkel of making “a catastrophic mistake” by accepting desperate refugees from the Middle East and said Germany was using European integration as a vehicle for its own interests.

Mr. Trump’s critiques of the European Union are shared by many Europeans. But he is wrong to suggest that the United States has no interest in the community’s survival. In addition to making war between its great powers unthinkable, European integration has helped consolidate democracy and the protection of human rights in countries across the continent, from Portugal to Romania. If it broke up, more nations would drift into the corrupt, autocratic orbit of Russia.

As for NATO, Mr. Mattis was right to tell the Senate Armed Services Committee that it “is the most successful military alliance in modern world history.” It has greatly magnified U.S. power and global influence, even when its members were underspending on their military forces. Without it, the West would have no effective way to contain Russian neo-imperialism.

Ms. Merkel said she will wait to see what Mr. Trump does when he is in office. It’s probably naive to hope that he will modulate his rhetoric. But Americans who value their country’s place in the world, including Mr. Trump’s Cabinet members, should do their best to ensure that he does not act on it. Once destroyed, the West’s alliances will not be easily rebuilt.