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The Supreme Court struck down Donald Trump’s sweeping “emergency” tariffs on Friday, ruling 6–3 that they far exceed what federal law allows. With its decision in Learning Resources v. Trump, the court wiped out Trump’s signature economic agenda, a withering rebuke to a president who has insisted that these tariffs are foundational to the success of his second term. Chief Justice John Roberts’ opinion for the court sends the blunt message that Trump should not expect SCOTUS to rubber-stamp all of his expansions of executive power, no matter how much political pressure he puts on the justices. This rejoinder may be surprising given the Republican-appointed supermajority’s previous tolerance for the president’s assertions of king-like authority. But as Roberts’ crisp, confident opinion explains, allowing the president to impose taxes unilaterally—at least without clear congressional authority—is an existential threat to the very “existence and prosperity” of the nation.

In truth, Trump’s tariffs were always on shaky legal ground, no matter how confidently the White House insisted they were permissible. The president claimed the freedom to impose tariffs on any nation, of any amount, for as long as he wished, based on the International Emergency Economic Powers Act, or IEEPA. But IEEPA does not mention tariffs, duties, taxes, or anything else that would hint at Congress’ desire to delegate tariff authority to the executive branch. Instead, it allows the president to “regulate” foreign “importation” to “deal with” an “unusual and extraordinary threat” abroad. Trump’s Justice Department insisted that he could “regulate” “importation” by slapping any tariff on any country he wanted. And it claimed two different “emergencies” that justified these duties: a long-standing trade imbalance with many other nations, and the smuggling of fentanyl into the United States.

Roberts—joined by Justices Neil Gorsuch, Amy Coney Barrett, Sonia Sotomayor, Elena Kagan, and Ketanji Brown Jackson—rejected this interpretation. All six........

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