Beyond projecting military strength and pledging unity, a more pressing theme has emerged for next week’s NATO summit: Keep President Trump happy.

As leaders prepare to meet for the annual forum starting on Tuesday, U.S. allies have watered down their public support for Ukrainian membership and drafted a policy communiqué as short as five paragraphs to keep the American leader on board. The meeting itself, in The Hague, will open and close in under two days — a timeline designed to keep it devoid of drama.

“No one wants to say no to Trump,” said Mujtaba Rahman, who analyzes Europe for the Eurasia Group. Asked on Wednesday whether the Iran-Israel war would prompt him to skip the meeting, Mr. Trump told reporters that he still planned to attend.

In any case, his influence is certain to loom over the gathering.

It has already driven an effort by NATO’s secretary general, Mark Rutte, to increase military spending by each of the alliance’s 32 members to meet a figure suggested by Mr. Trump. He has demanded it be raised to 5 percent of each country’s gross domestic product, up from the current level of 2 percent. Mr. Rutte has proposed widening the definition of military spending to help meet that objective.

The new benchmark would include 3.5 percent of G.D.P. on core defense spending — weapons, capabilities, troops — and the rest on what NATO calls “defense and security-related investment, including in infrastructure and resilience.”

In the weeks since Mr. Rutte’s idea gained steam, its details, and shortcomings, have become clearer, according to officials and experts. The timeline to increase spending may be different for everyone, and officials are confused about the requirements. Even if countries do allocate the sums, European and even American defense industries may not be able to absorb the money or deliver in a timely fashion.