US-Canada Trade War Enters Novel Legal Territory with Section 338 and USMCA Bypass

The US-Canada trade dispute escalated significantly in the final week of August 2026, with the United States' 50% Section 338 tariffs on approximately $20 billion of Canadian goods taking effect August 22, followed by Canada's announcement of dollar-for-dollar retaliatory tariffs on over 700 US products effective September 8. Section 338 of the Tariff Act of 1930 — invoked for the first time in US history to impose tariffs — permits the president to impose duties up to 50% against countries deemed to have discriminated against US commerce, with no investigation requirement and no statutory expiration date. The tariffs apply even to CUSMA-compliant goods, effectively bypassing the foundational assumption of the US-Mexico-Canada Agreement. Legal experts have flagged Section 338 as a potentially vulnerable authority in US courts, noting that the same Supreme Court that struck down IEEPA tariffs in February 2026 may scrutinize the breadth of executive tariff authority. Cross-border counsel should map client supply chains against the September 8 Canadian retaliation list — which covers steel, dairy, appliances, agricultural equipment, pulp and paper, electronics, and automotive components — before the effective date.