Those who do like to speak on behalf of Mr. Trump, though, present unorthodox foreign policy as similar to Richard Nixon’s. But what I could never figure out in these conversations is who plays Henry Kissinger to Mr. Trump’s Nixon. (Mr. Kissinger was, naturally, on my mind.)

Unlike government officials, the president’s critics in the think-tank world and the media seem eager to cavort with Europeans. It is a sort of never-ending psychoanalysis, in which it is not easy to figure out who is the patient and who is the analyst. As I met with dozens of such people around Washington, I heard the same things: Mr. Trump is at best an accidental president; he is a minority president; he was elected by the Russians; at some point soon (though not soon enough) he will be cast out of the White House. Behind this talk is a combination of anxiety and hope — anxiety about what Mr. Trump has broken and hope that as soon as he’s gone everything will return to normal.

That part is familiar. I’ve heard a similar hope in Europe, too. In most European capitals, policymakers and the chattering classes want to believe that before too long, Mr. Trump will be gone and the world order — including the close alliances between Europe and the United States — will return.

But here’s the dirty secret that I learned in my three months in Washington: That’s not true. The world will not boomerang back to where it was even if Democrats take back the White House in 2020, and not simply because even if Mr. Trump will be gone, many of the Trumpian leaders in power around the world will remain. Many of the changes Mr. Trump has ushered into America’s foreign policy will remain long after he’s gone. When it comes to America’s role in the world, he could end up being more consequential than George W. Bush or Barack Obama. The Trump moment in the end may resemble the Truman moment, when over a short period America dramatically changed its views of the world.

This may be hard for Europeans to swallow, but it’s the message I am bringing back with me from Washington. The post-Trump world will not be the pre-Trump world.