This chapter asserts that States placing themselves under the umbrella of nuclear-weapon States may not join the 2017 Treaty on the Prohibition of Nuclear Weapons without being in breach with one of its core provisions. The author considers four questions: How did the Treaty come to include a prohibition on threatened use? What does the prohibition mean for threatened self-defensive use of nuclear weapons under jus ad bellum? Does the prohibition cover nuclear deterrence? Does threatening to use nuclear weapons include threatening to have these weapons used on one’s behalf by its nuclear-armed ally? Whilst promoting universal adherence clearly coheres with the Treaty’s object and purpose, it is doubtful whether such considerations warrant a narrow construal in the hope that umbrella States would accede to the treaty without having to abandon their dependence onextended nuclear deterrence.