Iran’s pursuit of nuclear technology continues to place a major strain on US–Iranian relations, with many US decision makers still sceptical of Iran’s claims that its uranium-enrichment program is aimed only at providing fuel for civilian purposes, not at developing nuclear weapons capability. In spite of the diplomatic progress made to date, the conclusion of a comprehensive agreement resolving the issue remains elusive, with powerful elements in both states resistant to any compromise, and the United States’ key regional allies, Israel and Saudi Arabia, strongly opposed to the Obama administration’s decision to negotiate with the government of Hassan Rouhani. Consequently, a US attack on Iran in order to (at least) severely delay Iran’s nuclear program remains a distinct possibility. After outlining the causes of the current situation, and noting the extent to which both Iran and the United States have disregarded their obligations under the Treaty on the Non-Proliferation of Nuclear Weapons, this article considers the lawfulness of a potential US military strike against Iran, examining in detail relevant international legal rules governing the use of force. The conclusion reached is that such a preventive use of force would be — and should remain — illegal and that adherence to their respective legal obligations still offers the best way forward for both countries.