Costco has filed a major lawsuit challenging President Trump’s 2025 tariffs as unconstitutional and unauthorized by law. This detailed legal analysis explains how the case tests whether the International Emergency Economic Powers Act (IEEPA) gives presidents authority to impose tariffs, or whether that power belongs exclusively to Congress. With two federal courts already ruling these tariffs unlawful and the Supreme Court poised to decide, this case has profound implications for separation of powers, emergency authority limits, and who controls American trade policy. Learn why Costco needed its own lawsuit despite existing precedent, how customs liquidation deadlines forced urgent action, and what this means for constitutional checks on executive power.