On August 10, 2018, President Trump invoked Section 232 of the Trade Expansion Act of 1962 to issue Proclamation 10170, imposing a 25 percent tariff on imported steel. This national security rationale became the legal foundation for one of the administration's most consequential trade interventions. The proclamation applied broadly across nearly all trading partners, though bilateral negotiations produced exemptions for certain countries including Canada, Mexico, and South Korea. The mechanism was executive action without requiring congressional approval, leveraging Cold War-era statutory language that granted the president authority to adjust imports when deemed necessary for national defense.

The economic consequences rippled through multiple industrial sectors. Steel-consuming manufacturers in automotive production, appliance manufacturing, construction, and heavy machinery faced immediate cost pressures. A steel-dependent automaker, for instance, confronted significantly higher input costs that ultimately pressured wage growth and employment decisions. Consumers absorbed these effects indirectly through higher prices on vehicles, washing machines, refrigerators, and construction materials. Small manufacturers reliant on imported specialty steel faced steeper competitive disadvantages against larger competitors with greater pricing power.

This action marked an opening salvo in the Trump administration's broader trade strategy, establishing the precedent for using national security proclamations to implement tariffs. The 2018 steel tariff preceded subsequent aluminum tariffs and eventually broader China-focused trade actions. The pattern continued into the second Trump administration, as evidenced by the Continuation of National Emergency on Trade Deficits announced in March 2026, which preserved executive authority for ongoing tariff implementations. The Suspension of Duty-Free De Minimis Treatment for All Countries in February 2026 further escalated the tariff apparatus by eliminating exemptions on small-value shipments, demonstrating sustained commitment to tariff-based trade policy.

Legal challenges questioned whether national security provided legitimate justification for tariffs imposed primarily for economic competition rather than defense. The proclamation's constitutionality faced scrutiny regarding the scope of executive authority under Section 232, though courts largely upheld the president's discretion in applying the statute. No comprehensive congressional override materialized, leaving the tariff structure substantially intact through subsequent administrations.