The basic rationale is sound, whether or not Mr. Trump will decide to remove US nukes from Germany, Belgium and Italy. Many Ukrainians will admit that giving up the country’s nuclear arsenal in the 1990s was a tragic mistake, setting the stage for Russian interference and aggression in the years to come.

There is no sugarcoating the situation for Europe: Mr. Trump will not be “tough” on Russia, nor will he be interested in strengthening NATO. The former president called the alliance obsolete and has mused about leaving it. Forget “adults in the room” — the prospective Trump administration will be staffed far more heavily by sycophants and Trump loyalists than by traditional Republicans.

The bipartisan bill passed last year that supposedly prevents US presidents from withdrawing from the alliance without either the Senate’s approval or an act of Congress is legally hollow. The threat to the alliance is not America’s formal withdrawal but rather the possibility that a future president would simply choose not to come to the defense of an ally under attack and invoking Article 5.