This paper was written with a one-shot prompt (from Kevin Bryan) on o3 Deep Research, no iteration, 10 minutes of thinking. The Tariff Act of 1890, better known as the McKinley Tariff, was a pivotal episode in U.S. trade policy, dramatically raising import duties to near-record levels. This paper provides an analysis of the McKinley Tariff by integrating historical evidence with insights from modern international trade theory. We revisit the economic and political debates of the 1890s using contemporary trade models-including models of heterogeneous firms (Melitz, 2003), Ricardian comparative advantage in general equilibrium (Eaton and Kortum, 2002), and other new trade theory advances-to re-evaluate the tariff's impacts. Historical data on trade flows, tariff rates, and industry output are analyzed alongside contemporary accounts to assess the short-and long-run effects of the tariff. We find that while the McKinley Tariff accelerated the development of certain industries (notably tinplate production) and was implemented in an era of changing comparative advantage for the United States, its overall welfare effects were mixed and likely negative when evaluated with modern trade metrics. The tariff's protective gains to manufacturers came at the cost of higher prices for consumers and implicit burdens on agricultural exporters. However, consistent with modern trade models, the United States' large market power meant some tariff incidence was borne by foreign exporters. The paper concludes by drawing parallels between the McKinley Tariff episode and contemporary trade policy tensions, including recent U.S.-China tariff disputes and debates over protectionism in the global trading system.