President Donald Trump’s planned tariffs on steel and aluminum threaten a world-trading regime already battered by mounting protectionism and its struggle to tame China’s state-driven capitalism.

Mr. Trump is expected to sign a proclamation this week outlining his plan to impose the tariffs, which have angered close U.S. allies. European Union officials have suggested they could impose up to $3.5 billion in retaliatory tariffs on U.S. products almost immediately.

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