European leaders vowed on Tuesday to retaliate after President Trump announced that he would raise tariffs on aluminum and steel to 25 percent, even as they held out hope for a potential deal that could stave off painful effects for a weakened economy.

European Union officials have argued that Europe and the United States are deeply intertwined and that igniting a trade war would hurt both sides. But they have also been quietly preparing to hit back against possible U.S. tariffs, and officials have said they are ready to enact countermeasures if no trade solution can be found.

“Unjustified tariffs on the E.U. will not go unanswered — they will trigger firm and proportionate countermeasures,” Ursula von der Leyen, the president of the European Commission, the European Union’s executive arm, said in a statement.

Maros Sefcovic, the E.U. commissioner charged with trade and economic security, told members of the European Parliament on Tuesday

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