Once dominant, the U.S. finds itself isolated at the G20

The tariffs could very well provoke a global trade war.

European officials here reacted astringently, threatening to retaliate. “We will respond with countermeasures if need be, hoping that this is not actually necessary,” the European Commission president, Jean-Claude Juncker, said on Friday. “We are prepared to take up arms if need be.”

Targets could include American whiskey imports. “I don’t want to tell you in detail what we’re doing,” Mr. Juncker said. “But what I would like to tell you is that within a few days — we won’t need two months for that — we could react with countermeasures.”

The Italian prime minister, Paolo Gentiloni, warned that new protectionist trade measures could bring “contagion” that would slow the growth of the world economy.