Prime Minister Mark Carney of Canada began by wishing President Trump a happy birthday. He emphasized the importance of U.S. leadership in the Group of 7 alliance, which is meeting in Alberta, Canada.

But after seven minutes of questioning by journalists during which Mr. Trump complained about Russia’s absence at the summit and attacked Democrats over immigration policies, the host of the summit had heard enough.

He took a step forward and into the center of the frame and effectively stopped the questioning, preventing the American president from saying more.

With war raging in the Middle East and U.S. tariffs hammering his own country’s economy as well as global trade, Mr. Carney was intent on limiting the chances of a Trump-related derailment of the gathering.

“If you don’t mind, I’m going to exercise my role, if you will, as G7 chair, since we have a few more minutes with the president and his team and then we actually have to start the meeting to address some of these big issues,” Mr. Carney said. “So, merci beaucoup.”

With that, the press was rapidly escorted out of the room.

The brief moment at the start of the gathering provided a window into a daunting challenge for world leaders entering the summit: Just what is the best way to manage Mr. Trump on the global stage?