Donald Trump prepared for this week’s NATO summit by doing what no president had done before — making a case that the alliance is a bad deal for the American people. Last week in Great Falls, Mont., he said that he had told Chancellor Angela Merkel of Germany, “I don’t know how much protection we get by protecting you.” Mr. Trump has been even tougher on the European Union, branding it “as bad as Nafta” and adding, “Sometimes our worst enemies are our so-called friends.”

I have visited four European countries during the last two weeks, and it has been shocking to see how far from grace the United States has fallen in the eyes of its allies. European leaders point to Mr. Trump’s support for anti-democratic populists in Hungary, Poland and Italy. They view his recent Twitter attack on Ms. Merkel as a transparent attempt to push her from office.

Many fear he may now remove American sanctions against Moscow over its occupation of Crimea after his meeting with President Vladimir Putin of Russia in Finland next week. Confidence in Mr. Trump has plummeted so much that the German foreign minister, Heiko Maas, recently grouped “Donald Trump’s egotistic policy of America First” along with Russia and China as global concerns.

None of this, of course, is likely to disturb Mr. Trump, who remains steadfast in his belief that whatever benefits the United States gained from the trans-Atlantic alliance in the past, the country no longer profits. But he’s wrong — there are compelling reasons that NATO in particular will be a distinct advantage for America’s security far into the future.