The breakdown of the arms control regime bolstered by the ongoing arms race in the development of new weapon and delivery systems is of grave concern to the international community. Under the concept of flexible deterrence, nuclear weapons upgrades and modifications to lower yields are supposed to expand options for their deployment. The question that arises is, would such use on the modern battlefield be legal under the law of armed conflict, or would it constitute a severe violation of this body of law. The authors argue that most customary principles and rules of the law of armed conflict would not necessarily deem the use of nuclear weapons illegal. However, such use would gravely violate the fundamental principle of prohibition of weapons of a nature to cause superfluous injury or unnecessary suffering. The authors examine medical data gathered from Hiroshima and Nagasaki, including data related to the long-term health effects of the nuclear bombings. Ionizing radiation emitted during a nuclear explosion serves no military purpose; it offers no military advantage while unnecessarily aggravating the suffering of affected combatants. While the use of nuclear weapons might not necessarily violate other fundamental principles applicable during the conduct of hostilities such as proportionality, distinction or precautions, depending on the method of deployment, the use of nuclear weapons would violate the customary rule of prohibition of employment of weapons of a nature to cause superfluous injury or unnecessary suffering.