The head of NATO wants the alliance’s annual summit meeting next month to be short and sweet, European officials and diplomats say, to prevent the kind of open disunity over Ukraine that marred the gathering two years ago.

But two things could leave the alliance’s secretary general, Mark Rutte, presiding over a more difficult event: President Trump, who is expected to attend, is at odds with allies over Ukraine’s future. And Ukraine itself wants to join NATO, an outcome Mr. Trump opposes.

The NATO summit, set for June 24-25 in the Netherlands, comes as the United States is retreating from maintaining primary responsibility to protect Europe, creating significant uncertainty about the continent’s security. Trump administration officials have warned their European counterparts that major changes in American troop rotations are imminent even as they seek to reassure allies that the United States is committed to NATO.

At the same time, Mr. Trump is winding down support for Ukraine, has abandoned efforts to reach a cease-fire in the war and seeks to normalize relations with Russia.

Given all that, Ukraine’s role in the summit is uncertain, not least because of Mr. Trump’s dismissiveness toward President Volodymyr Zelensky, who is also expected to attend.

It is not even clear that Mr. Zelensky will be invited to the main opening dinner.

“I fully expect Zelensky to be at The Hague,” Matthew G. Whitaker, the new American ambassador to NATO, said at a conference this month in Tallinn, Estonia. “In what capacity, we’re discussing.”