Donald Trump meets NATO and EU leaders on Thursday in the city he once derided as a "hellhole", with nervous allies hoping for a strong show of commitment from the US president.
The billionaire leader, on his first foreign tour, is getting a red-carpet welcome from the same organisations he ridiculed on the campaign trail in remarks that sparked fears for transatlantic ties.
But he was also greeted by large protests as he arrived in Brussels on Wednesday amid tight security for high-stakes meetings overshadowed by the concert bombing in Manchester, Britain, claimed by the Islamic State group.
Leaders will be hoping to convince Trump of the lasting relevance of the Brussels-based European Union and the North Atlantic Treaty Organisation, together widely credited with keeping peace in Europe after World War II.
Trump dismissed NATO as "obsolete" for failing to tackle Islamist terror and focussing on Russia instead, while he alarmed the EU by backing Britain's exit from the 28-nation bloc.
But the mercurial Trump has since backed away from his comments, and hopes are high in Brussels that he will finally make a display of solidarity.
EU President Donald Tusk, who will meet Trump along with European Commission chief Jean-Claude Juncker, tweeted that "I'll aim to convince POTUS that euro-atlanticism means the free world co-operating to prevent (a) post-West world order".
Trump set out his stall as he met Belgian Prime Minister Charles Michel shortly after his arrival on Wednesday night, saying that the most important issue was terrorism after the "horrible situation" in Manchester.
"When you see something like what happened a few days ago you realise how important it is to win this fight. And we will win this fight," said Trump, fresh from meeting Pope Francis at the Vatican.
There was no sign of tension with Michel over Trump's calling Brussels a "hellhole" because of Muslim immigration in January 2016, two months before the city was struck by Islamic State suicide bombings.
However at least 9,000 people marched through Brussels on Wednesday night waving blond-haired effigies of the reality TV star president and flags saying "Trump not welcome".
Further protests were planned on Thursday, when Trump will first meet with EU President Tusk and Commission chief Juncker at the bloc's headquarters.
Tusk and Juncker will tell the US president that since last year's shock Brexit vote, the EU is "in a completely different place" after populist candidates lost in France and the Netherlands, a senior EU official said.
Trump is then set to dine with French President Emmanuel Macron, whose recent victory over far-right leader Marine Le Pen has been seen as a beacon of hope by Brussels.
The NATO summit after that will be full of pomp and symbolism, with the keen-to-impress alliance showing off its new $1.2 billion headquarters and staging a flypast.
At a ceremony with German Chancellor Angela Merkel, Trump will unveil a memorial to the Sept. 11, 2001 attacks featuring part of the destroyed World Trade Center, while Merkel does the same for a fragment of the Berlin Wall.
In return, allies rattled by a resurgent Russia will hope for a public display of commitment from Trump to Article 5, the alliance's one-for-all collective defence pledge.
While this has been triggered only once, by the US after 9/11, Trump has said that any future use might depend on whether a NATO member had met its spending commitments.
NATO Secretary General Jens Stoltenberg said on the eve of the summit that he was confident of Trump's commitment.
He also urged allies to "step up" and join the US-led anti-IS coalition "not least because of the attack we saw in Manchester", and said they would deliver yearly spending plans.
The alliance is now set to formally join the coalition after France, Germany and Italy dropped their opposition, diplomatic sources said.
Trump's wife Melania, meanwhile, is set to visit a museum dedicated to the surrealist artist Rene Magritte and a leading leather store while in Brussels.
Trump's sweep through Saudi Arabia, Israel and the Vatican, centres of three of the world's main religions, is being followed by Brussels and a trip to Italy for the G7 leaders summit on Friday.
On Wednesday Trump said he was "more determined than ever to pursue PEACE in our world" following a "fantastic" meeting with Pope Francis.
The high-profile trip has diverted attention from Trump's domestic pressures over alleged campaign collusion with Russia.