The allies are likely to endorse a goal of spending 5% of their gross domestic product on their security, to be able to fulfil the alliance's plans for defending against outside attack.
Still, Spain has said it cannot, and that the target is "unreasonable".
President Donald Trump has said the US should not have to.
Slovakia said it reserved the right to decide how to reach the target by NATO's new 2035 deadline.
"We are not living in happy land after the Berlin Wall came down. We are living in much more dangerous times and there are enemies, adversaries who might want to attack us," NATO Secretary-General Mark Rutte said on Tuesday the summit in The Hague.
"We have to make sure that we defend our beautiful way of life and systems and our values."
Before the two-day meeting, Britain, France and Germany committed to the five per cent goal.
Host country the Netherlands is also onboard. Nations closer to the borders of Ukraine, Russia and its ally Belarus had previously pledged to do so.
Trump's first appearance at NATO since returning to the White House was supposed to centre on how the US secured the historic military spending pledge from others in the security alliance - effectively bending it to its will.
But in the spotlight instead now is Trump's decision to strike three nuclear enrichment facilities in Iran that the administration says eroded Tehran's nuclear ambitions, as well as the president's sudden announcement that Israel and Iran had reached a "complete and total ceasefire".
Ukraine has also suffered as a result of that conflict.
It has created a need for weapons and ammunition that Kyiv desperately wants, and shifted the world's attention away.
Past NATO summits have focused almost entirely on the war in Ukraine, now in its fourth year.
Still, Rutte insists it remains a vital issue for NATO, and the allies can manage more than one conflict.
"If we would not be able to deal with ... the Middle East, which is very big and commanding all the headlines, and Ukraine at the same time, we should not be in the business of politics and military at all," he said.
"If you can only deal with one issue at a time, that will be that. Then let other people take over."
Ukrainian President Volodymyr Zelenskiy arrived in The Hague for a series of meetings, despite his absence from a leaders' meeting aiming to seal the agreement to boost military spending.
It's a big change since the summit in Washington in 2024, when the military alliance's weighty communique included a vow to supply long-term security assistance to Ukraine, and a commitment to back the country "on its irreversible path" to NATO membership.
In a joint tribune on the eve of the 2025 summit, French President Emmanuel Macron and German Chancellor Friedrich Merz said they backed US peace efforts that should preserve Ukraine's sovereignty and European security.
"For as long as the current trajectory lasts, Russia will find in France and Germany an unshakeable determination. What is at stake will determine European stability for the decades to come," they wrote in the Financial Times newspaper.
