“The only security guarantee for Ukraine is NATO membership,” she said, citing the protection membership affords her own small country. “We don’t have war here because we are members of NATO,” she said.

Another benefit, she said, is that having Ukraine inside NATO would be “cheaper, much cheaper” than making it a militarized hedgehog for the next 50 years.

The counterargument, held widely in Washington and Western Europe, is that NATO cannot accept a country at war over disputed territory, and that such a move could push Russia to escalate further, even with nuclear weapons, before Ukraine could enter the alliance. But so far, Russian threats of escalation have proven hollow.

For now, ahead of the summit, NATO countries are preparing a medium-term plan of pragmatic military assistance for Ukraine, including guaranteed arms supplies and further integration into NATO’s world. But Mr. Zelensky wants a political promise he can take home.

Still, if the war does not in the end produce large-scale Russian withdrawal and defeat, what could prove convincing to Mr. Zelensky and Ukrainians — giving any peace talks the most leverage — would be NATO membership, behind solidified cease-fire lines, perhaps patrolled, Mr. Heisbourg suggests, by a coalition of peacekeeping forces from NATO and other countries, like India or even China.