The law governing nuclear weapons no longer describes the regime it governs.The Non-Proliferation Treaty does not regulate deterrence. The International Court of Justice examined the question in 1996 and stopped short of resolving it. The Treaty on the Prohibition of Nuclear Weapons prohibits without settling how it sits alongside the older instruments.
This book reads them together rather than apart. Using the customary rules of treaty interpretation, it derives a test from the regime itself that reconciles the instruments as a single legal order and makes the regime a legal condition on any claim of lawful use.