The offer by Poland’s president to host an American military base and call it “Fort Trump” is certainly going to test the debate over the North Atlantic Treaty. On the one hand, Democrats and McCain Republicans have been railing about Mr. Trump’s supposed retreat from global leadership. On the other hand, many fear the illiberal nationalism of which President Duda, who offered “Fort Trump,” is an avatar.

Mr. Duda sprang the business about “Fort Trump” at a joint press conference Tuesday at the White House. He is here in advance of the double jubilee of the restoration in 1918 of Poland’s sovereignty. It’s not the first time Poland has pressed the idea of hosting an American base; Foreign Minister Radek Sikorski, albeit now out of office, pointed out — compellingly so, in our view — the logic of it for years.

Mr. Duda seemed to appreciate the humor in the sobriquet of “Fort Trump,” as did Mr. Trump. It’s hard to imagine the Russians will see the humor in it. It was only two generations ago that the Soviet Union occupied Poland, which was a member of the Warsaw Pact. The danger from Russia, though, would be part of the logic of moving one of our bases into Poland in the first place.

That and the issue of burden sharing, which Mr. Trump has made a centerpiece of his complaints about NATO. Only four members of the North Atlantic alliance — America, Greece, Britain, and Estonia — pay a greater share of their GDP on defense than Poland, which last year was a hundredth of a percentage point below the goal of 2% of GDP. Mr. Trump acknowledged the point on Tuesday.

Mr. Trump also acknowledged Mr. Duda’s preparedness to make a contribution to the cost of a base. Mr. Trump spoke Tuesday of $2 billion or more. That’s not a lot of money in the context of our spending on overseas bases. Neither, though, is it anything to sneeze at on a continent where American outlays, by far the largest in NATO as a percentage of GDP, seem to be taken for granted.

As the idea of a major American base in Poland, even without invoking “Fort Trump,” starts to percolate in Washington, it is going to be hard to ignore the question of Polish democracy. It will come up in Congress, which has the sole constitutional grant of power to raise an army in the first place. Neither, though, will anyone be able to ignore Poland’s astounding history and the folly it has taught of leaving the country undefended.