President Donald Trump vowed to keep fighting terrorism, calling the attack in Manchester a reminder of the importance of the battle.

“[T]he top problem right now is terrorism,” Trump said after landing in Brussels, during a meeting with Prime Minister Charles Michel of Belgium.

The president added that under his Generals, the fight was “doing very well,” but described the terrorist attack in Manchester as a “horrible” and “unthinkable” situation.

“When you see something like what happened a few days ago, you realize how important it is to win this fight,” he said. “And we will win this fight.”

Trump will meet with NATO allies in Brussels to discuss the future of the alliance and the ongoing struggle against terrorism.

Speaking with reporters on Air Force One, Secretary of State Rex Tillerson said that NATO should join the coalition against ISIS.

“They’ve been an observer. But they’ve become more and more engaged in the actual fight to defeat ISIS,” he said.