NATO — made up of 32 European and North American allies committed to defending each other from armed attack — will stress strength through solidarity as it celebrates its 75th anniversary during the summit starting Tuesday.
Event host Biden, who pulled allies into a global network to help Ukraine fight off Russia's invasion, has called the alliance the most unified it has ever been.
But behind the scenes, a dominant topic will be preparing for possible division, as the power of far-right forces unfriendly to NATO grows in the US and other countries including France, raising concerns about how strong support will stay for the alliance and the military aid that its members send to Ukraine.
At the presidential debate, Biden asked Trump: "You're going to stay in NATO or you're going to pull out of NATO?"
Trump tilted his head in a shrug.
Biden's poor debate performance set off a frenzy about whether the 81-year-old president is fit for office or should step aside as the Democratic presidential candidate.
There is some nervousness among NATO allies about Joe Biden's re-election race. (AP PHOTO)
Even before the debate, European governments were deep in consultations on what they could do to ensure that NATO, Western support for Ukraine and the security of individual NATO countries will endure should Trump win back the presidency in November and temper US contributions.
Some Americans and Europeans call it "Trump-proofing" NATO — or "future-proofing" it when the political advances of other far-right political blocs in Europe are factored in.
This week's summit, held in the city where the mutual-defence alliance was founded in 1949, was once expected to be a celebration of NATO's endurance.
Now, a European official said, it looks "gloomy".
There are two reasons for the gloom: Russian advances on the battlefield in the months that Trump-allied congressional Republicans delayed US arms and funding to Ukraine.
And the possibility of far-right governments unfriendly to NATO coming to power.
The official spoke to reporters last week on condition of anonymity to discuss private conversations among governments.
Rachel Rizzo, a senior fellow on NATO with nonpartisan think tank the Atlantic Council, says she has a blunt message for Europeans: "Freaking out about a second Trump term helps no one".
For allies at the summit, she said, the key will be resisting the temptation to dwell on the details of unprecedented events in US politics and put their heads down on readying Western military aid for Ukraine and preparing for any lessening of US support.
Trump, who before and after his presidency has spoken admiringly of Russian President Vladimir Putin and harshly of NATO, often focuses his complaints on the US share of the alliance's costs.
Biden himself, as a US senator in 1997, warned that if there were any sense other NATO allies were "taking the United States for suckers, the future of the alliance in the next century will be very much in doubt."
The 1991 collapse of the Soviet Union lulled the West into thinking the Russian threat had been neutralised, leading to military spending cuts.
Now, NATO allies are bolstering their forces against any wider aggression by Putin, and a record 23 nations in NATO are meeting defence-spending goals.
Trump's former national security adviser, John Bolton, says Trump in a second term would work to get the US out of NATO.
Congress passed legislation last year making that harder, but a president could simply stop collaborating in some or all of NATO's missions.
