President Donald Trump is meeting with Belgian Prime Minister Charles Michel in Brussels.
President Trump said they will work together on "various problems. Number one is terrorism".
The president said that when you see something like the bombing this week in Manchester, you remember how important it is to "win the fight".
He added that "we will win. 100%".
President Trump is meeting with the prime minister before talks this week with Nato and European Union officials.
Meanwhile, thousands of protesters have gathered for a major anti-Trump demonstration in the Belgian capital.
The march was set to start in the early evening, just as President Trump was ending talks with Belgian government officials.
The demonstrators centred their protests on President Trump's environmental and immigration policies.
All civilized nations must join together to protect human life and the sacred right of our citizens to live in safety and in peace.— Donald J. Trump (@realDonaldTrump) May 23, 2017
The early arrivals at the Brussels North Station were shouting: "We don't want Trump! We don't want Trump!"
The road of the marchers was set to stay away from the US Embassy and Royal Palace where Mr Trump was holding meetings upon his arrival from Rome.