Newly declassified archival sources allow a reassessment of U.S.-Canadian diplomacy during the final months of John Diefenbaker’s government concerning Canada’s prospective acquisition of nuclear weapons in the wake of the October 1962 Cuban missile crisis. Scholars have traditionally argued that Canadian proposals for U.S. nuclear warheads to be supplied to Canada after the outbreak of an international emergency were unworkable. Diefenbaker has been deemed primarily responsible for his government’s collapse after personally fumbling the bilateral nuclear weapons talks. Drawing on previously unavailable primary documents, this article shows that the U.S. decision to reject Ottawa’s proposals was rooted in political, not military, imperatives. The article also demonstrates that U.S. officials waged a concerted campaign to undermine the Canadian government, most notably through the State Department’s unprecedented public rebuke of Diefenbaker’s nuclear weapons policy in late-January 1963.