The overriding focus of the gathering -- which kicks off with dinner hosted by the Dutch king -- has been on keeping Trump happy after his return to power sparked fears he could blow a hole in the seven-decade-old alliance.

From meeting his spending demand to limiting the participation of Ukraine's President Volodymyr Zelensky, everything is being done to try to avoid the chances of a damaging confrontation with the volatile leader.

But despite the careful planning the meeting still risks being overshadowed by the fallout from Trump's decision to strike Iran's nuclear facilities.

Central to the effort to keep Trump on board -- given his threat not to protect lower-spending NATO allies -- will be a pledge to satisfy his insistence that members cough up five percent of their GDP on defence.

To give him a headline victory, NATO's 32 countries have thrashed out a compromise deal to dedicate 3.5 percent to core military needs by 2035, and 1.5 percent to broader security-related areas like cybersecurity and infrastructure.

"This is a quantum leap that is ambitious, historic and fundamental to securing our future," said NATO chief Mark Rutte, who warns the alliance must be ready to confront the threat of a Russian attack within five years.

- Deep divisions -

"We must navigate this era of radical uncertainty with agility, speed," British Prime Minister Keir Starmer said Monday in announcing the UK's commitment to meet NATO's spending target.

In a joint Financial Times op-ed on the summit's eve, French President Emmanuel Macron and German Chancellor Friedrich Merz likewise argued that Europe must rearm "not because someone asks us to, but because we are clear-eyed and owe it to our citizens to do so".

In the run-up to the meeting, Spain had sparked fears of undermining the alliance's carefully choreographed unity by refusing to commit to the headline figure of five percent.

But Rutte stressed Spain had not been granted an "opt-out" from the pledge.

"The summit will be historic," US NATO ambassador Matthew Whitaker said.

"This summit is really about NATO's credibility and we are urging all of our allies to step up to the plate and pay their fair share for transatlantic security."

But while the promise of more spending looks like it could win Trump over, deep divisions remain over the approach to Europe's key security issue: Russia's war in Ukraine.

Since storming back to power Trump has upended the West's approach to the three-year conflict by turning his back on Kyiv and opening the door to closer ties with Moscow.

- Zelensky downgraded -

Zelensky, who had an infamous Oval Office bust-up with Trump, has been downgraded from the central role he played at recent NATO gatherings.

This time he is set to attend the king's dinner, meet EU leaders and attend an industry forum -- but he will not hold formal talks with the alliance's 32 leaders.

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

The French and German leaders, in a message aimed at Trump, said they would jointly press for a ceasefire in Ukraine and ramped-up pressure on Russia "including through sanctions" while in The Hague.

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

Allies have also had to push hard just to get Washington to name Moscow as a "threat" in a summit declaration set to be released when NATO leaders hold their main session on Wednesday.

The push to make sure the United States remains on the same page comes as Washington is weighing whether to pull forces from Europe as it focuses more on the threat from China.

The Pentagon is set to announce the results of a review of its global deployments later in the year -- but diplomats at NATO say they don't expect Trump to make any announcements in The Netherlands.