President Trump signed Proclamation 9705 on March 8, 2018, invoking Section 232 of the Trade Expansion Act of 1962 to impose a 25 percent tariff on imported steel and a 10 percent tariff on imported aluminum. The proclamation granted limited exemptions to Canada and Mexico while applying the duties to nearly all other trading partners. This mechanism, which allows the president to restrict imports deemed necessary for national defense, became a cornerstone of the administration's trade policy and would be repeatedly invoked in subsequent years to justify additional tariff regimes.

The tariffs rippled through interconnected supply chains affecting millions of American businesses and consumers. Steel-consuming manufacturers in automotive, construction, appliance, and machinery sectors faced substantially higher input costs. A vehicle manufacturer purchasing domestic steel at the elevated tariff-inclusive price saw production expenses climb, costs that manufacturers ultimately transferred to consumers through higher vehicle prices. Construction firms bidding on projects suddenly faced uncertainty in material pricing. Small manufacturers without long-term supply contracts bore disproportionate burden. Consumer goods prices across appliances, tools, and countless other steel and aluminum-dependent products increased as the tariff costs flowed through retail channels.

These tariffs established a pattern of trade restrictions that would define the Trump administration's economic approach. The 2018 steel and aluminum duties preceded broader tariffs on Chinese goods and represented an early assertion of emergency powers for trade purposes. By 2026, the administration continued invoking national emergency authority for trade deficits, suspended duty-free de minimis treatment on all imports, and maintained selective tariffs—demonstrating how initial tariff actions created legal and bureaucratic precedent for expanded trade restrictions. The steel and aluminum tariffs effectively normalized using Section 232 authority for protectionist purposes, a tool that subsequent administrations could leverage.

Litigation challenges mounted quickly, with trading partners filing complaints through the World Trade Organization and American businesses seeking relief. The legal foundation proved resilient, however, as courts initially deferred to executive authority on national security determinations. Congressional efforts to restrain the tariffs proved limited, constrained partly by divided Republican support for trade protectionism.