Leaders from 32 countries are gathering in Washington for NATO’s 75th anniversary summit, but all eyes are on just one: President Biden.

With questions circulating over his mental and physical fitness to run for reelection, Biden will now be tested on a world stage.

At stake is not just Biden’s presidency but, potentially, the future of the transatlantic alliance that has kept relative peace in a large part of the world for more than half a century.

Biden has long prided himself on his skills in dealing with world leaders. And he will surely use the summit to emphasize what some analysts consider his finest foreign policy achievement: the unity forged among NATO members to support and aid Ukraine in its devastating war with an invading Russia.

Most leaders attending this annual meeting of the North Atlantic Treaty Organization are expected to praise Biden and his efforts.

Publicly, they will not criticize him. But the “Biden question” looms over the crowded, heavily guarded convention center where they are gathering.

Privately, diplomats say, several foreign officials have expressed concerns about Biden’s perceived frailties, citing occasional lapses in bilateral meetings or apparently confused statements.

For some, the issue is practically existential: If Biden loses to Donald Trump, a convicted felon who has harshly criticized NATO, a central transatlantic mutual defense pact that has governed world order since World War II may be in danger.

Trump has derisively dismissed the importance of NATO while showing deference toward and admiration of Russian President Vladimir Putin.

Administration officials dismiss any concerns among the mostly European leaders about Biden.

“These leaders know him, they’ve worked with him,” a senior administration official said, briefing reporters ahead of the summit. Speaking on condition of anonymity, the official said the leaders continue to have confidence in Biden.