Everything has been carefully choreographed at the gathering in The Hague to keep the volatile US president on board: from chopping back the official part of the meeting to putting him up overnight in the royal palace.

The strategy seemed to be working -- for now -- with Trump seeming keen to share the plaudits for a deal set to see the 32 countries commit to spending five percent of output on defence by 2035.

"It's a great victory for everybody, I think, and we will be equalised very shortly, and that's the way it has to be," said Trump, as European allies seek to catch up with US spending on defence.

"I've been asking them to go up to five percent for a number of years, and they're going up to five percent... I think that's going to be very big news," he said.

Hosting the meeting, NATO Secretary General Mark Rutte told reporters that Trump was in an "excellent mood" at the dinner hosted on Tuesday by King Willem-Alexander in his royal palace, and that the US leader appeared inspired by his hosts.

"The day begins in the beautiful Netherlands. The King and Queen are beautiful and spectacular people. Our breakfast meeting was great!" he posted on the Truth Social network.

Entering the meeting, leaders lined up to declare the summit's planned spending hike as "historic".

NATO allies say the increase is needed to counter a growing threat from Russia but also to keep Trump engaged, with the US leader long complaining that Europe spends too little on its own defence.

Belgian Prime Minister Bart De Wever said: "As Europeans, we should realise that our long break from history is over."

The continent needed to take responsibility for its own security "in a very difficult time", added De Wever.

The pledge divides up the spending into 3.5 percent of GDP on core defence costs -- just above the current US level -- plus another 1.5 percent to broader security-related areas such as cybersecurity and infrastructure.

- 'Totally committed' -

With that deal all but clinched, attention turned to the basic foundation of the alliance -- its mutual defence clause that says an attack on one is an attack on all.

Trump rattled his allies by appearing to cast some doubt on the validity of this promise -- known as Article Five of the NATO treaty -- telling reporters on the way to The Hague that it "depends on your definition. There's numerous definitions of Article Five."

But Rutte played down the comments, saying he believed that Trump and the United States were still "totally committed" to Article Five.

And when pressed about the collective pledge while meeting Rutte on Wednesday, Trump said: "We're with them all the way."

Britain's Prime Minister Keir Starmer dodged multiple questions seeking clarity over Washington's stance.

"We live in a very volatile world and today is about the unity of NATO, showing that strength," said Starmer.

In an earlier message, probably not designed for public consumption, Rutte heaped flattery on Trump, praising him for bringing everyone on board for the spending hike.

"Europe is going to pay in a BIG way, as they should, and it will be your win," Rutte wrote in a gushing, caps-filled missive to Trump, who promptly posted it on social media.

"You are flying into another big success in The Hague," wrote Rutte.

- 'Nice guy' -

Underpinning the leaders' discussions on defence was Russia's invasion of Ukraine, with Trump poised to meet the war-torn country's president Volodymyr Zelensky on the summit sidelines.

Zelensky is playing a less central role here than at previous summits, to avoid a bust-up with Trump after their infamous Oval Office shouting match.

But Trump described him as a "nice guy" and added that he was talking to Russian President Vladimir Putin about the war, saying: "I think progress is being made."

Rutte said that allies would send the message that support for Kyiv was "unwavering and will persist".

But despite his insistence that Ukraine's bid for membership remains "irreversible", NATO will avoid any mention of Kyiv's push to join after Trump ruled it out.

Hungary's Kremlin-friendly Prime Minister Viktor Orban was more categorical.

"NATO has no business in Ukraine," he said. "My job is to keep it as it is."

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