President Trump left the G7 after firming up a trade deal with the United Kingdom. But he hinted at roadblocks on other fronts, saying Tuesday that Japan was being "tough" in trade talks and the European Union has failed to offer a fair deal.

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...