The 2018 summit ended with Trump assailing his Canadian hosts on social media as he departed on Air Force One, saying he had instructed the U.S. officials who remained in Quebec to oppose the G7 joint statement endorsed by the leaders of Japan, France, the United Kingdom, Italy, Germany and, of course, Canada.

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...