On March 8, 2018, President Trump signed Proclamation 9740 invoking Section 232 of the Trade Expansion Act of 1962 to impose a 10 percent tariff on aluminum imports entering the United States. The proclamation took effect on March 23, 2018, with limited carve-outs for Canada and Mexico. This invocation of national security authority—a provision typically reserved for defense-related trade matters—marked an expansive interpretation of presidential tariff power that would characterize the administration's broader trade policy approach throughout its tenure.

The tariff's direct effects rippled across American supply chains and consumer budgets. Aluminum producers and refiners initially benefited from reduced foreign competition, but manufacturers dependent on imported aluminum faced immediate cost pressures. Beverage companies, automotive manufacturers, and construction firms—sectors consuming substantial aluminum volumes—passed increased material costs onto consumers. A can of soda, an automobile door panel, and an aluminum window frame all became more expensive as tariffs cascaded through production chains. American workers in aluminum-intensive industries faced potential layoffs if their companies relocated production or scaled back operations in response to higher input costs.

This action exemplified the national emergency framing that would become institutionalized in Trump trade policy, as reflected in the 2026 continuation of the national emergency on trade deficits—demonstrating how temporary emergency measures became permanent features. The aluminum tariff preceded similar actions on steel and other products, establishing a pattern of unilateral trade action that persisted across multiple administrations. Unlike the subsequent 2026 tariff terminations, which reversed some earlier actions, this specific aluminum tariff remained substantially in place, illustrating the durability of Section 232 impositions.

The proclamation faced limited immediate legal challenge compared to other tariff actions, though economists and trade associations consistently disputed the national security rationale for aluminum tariffs. The provision's broad executive discretion made judicial reversal difficult absent Congressional action to narrow Section 232 authority or explicitly reject the national emergency framework underlying these trade actions.