Trump railed against former Prime Minister Justin Trudeau after that Group of Seven summit and said he would yank the United States from a joint statement. The leaders fought over retaliatory tariffs on critical steel and aluminum.

If there is a shared mission at this year’s G7 summit, which begins Monday in Canada’s Rocky Mountains, it is a desire to minimize any fireworks at a moment of combustible tensions.

Israel’s strikes on Iran and Tehran’s retaliation, which appeared to catch many world leaders unawares, is the latest sign of a more volatile world as Trump seeks to withdraw the U.S. from its role a...