BUENOS AIRES — Treasury Secretary Steven Mnuchin[1] doubled down on the Trump administration’s tough talk on trade over the weekend even as his international counterparts warned U.S. tariffs could stunt global growth and usher in a new era of job-killing protectionism.

In a deja vu of previous summits, Mr. Mnuchin[2] found few allies at the two-day meeting of Group of 20 finance ministers and central bankers, many of whom openly labeled the recent tit-for-tat over tariffs U.S.-wrought “trade wars.”

But Mr. Mnuchin[3] declined to soften President Trump[4]’s thunder, which on Friday included the threat of an additional $500 billion in tariffs on Chinese goods, noting he had backed Mr. Trump[5]’s hard-line stance from the get-go.

“I worked with him during the campaign, so I’m not new to any of these economic issues; I helped develop these economic issues.” Mr. Mnuchin[6] told The Washington Times. “From my first meeting at the G-20 … I’ve tried to emphasize that this is about making sure that we have free and fair, two-way trade.”

Economic analysts say Mr. Mnuchin[7] did ease some fears sparked by his boss’ tweets over the weekend on another front, denying the U.S. government wants a weaker dollar to boost exports and softening Mr. Trump[8]’s criticism that China and other major trading partners are manipulating their currencies to help their own exporters.

The U.S. treasury secretary said anyone surprised by Mr. Trump[9]’s combative stance hasn’t been paying attention.

China, Mr. Mnuchin[10] insisted, is fully aware of U.S. demands to let American companies compete freely and expand U.S. exports to slash the current $370 billion trade deficit. But with his Chinese counterpart a no-show in Buenos Aires, the showdown quickly shifted to the brewing threat of a second trade war — with Europe.

Still angry over steel and aluminum tariffs and fearing possible duties on their car makers, European officials here toggled between indignation, threats of retaliation and — albeit flimsy — optimism that talks between Mr. Trump[11] and European Union President Jean-Claude Juncker this week may help turn things around.

Mr. Juncker’s economics commissioner, Pierre Moscovici, on Sunday tried to walk back comments by French Finance Minister Bruno Le Maire, who had warned that Europe would “refuse to negotiate [with Washington] with a gun to the head.”...

“We are of course ready to respond firmly; it’s not a question of a gun, but we are prepared,” Mr. Moscovici said. “But we’re also there to build bridges, and we hope that this meeting, which is — I can see that — [eagerly] awaited by our American partners, will be the beginning of something.”Between the lines, however, the Europeans gathered here acknowledged that the breakdown of U.S.-China trade talks did not play in their favor, but Mr. Moscovici

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