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.
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.
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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.
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 last 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.
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.
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