As President Trump’s assault on Iran enters its third week, European leaders are largely resisting his bellicose demands for help in reopening the Strait of Hormuz.

At the same time, they are trying to avoid irreparably damaging their relationship with the United States over their opposition to another war of America’s choosing.

To Mr. Trump, it should hardly be a difficult decision. He views Europe’s action — or inaction — in the face of the strait’s closure as a test of its commitment to the continent’s own security. Sending their navies for what he called a “very small endeavor” is the least that Europe’s presidents and prime ministers can do, Mr. Trump suggested over the weekend.

At an event on Monday at the White House, Mr. Trump complained that some European leaders were not showing their appreciation for everything that the United States had done to protect the continent.

“We have some countries where we have 45,000 soldiers, great soldiers, protecting them from harm’s way, and we have done a great job,” he said. “And well, we want to know, do you have any mine sweepers? ‘Well, we’d rather not get involved, sir.’”

The American president also issued a not-so-veiled warning in an interview with the Financial Times on Sunday, saying that “it will be very bad for the future of NATO” if European nations do not join the United States in its effort to reopen the vital waterway to tankers carrying oil, gas and fertilizer. At Monday’s event, he said: “I think we’re going to have some good help. And I think we’re going to be disappointed in some nations, too.”