The concept of nuclear nonproliferation was coined in a formal way at the beginning of the 1960s, though the Treaty on the Non-Proliferation of Nuclear Weapons, or Non-Proliferation Treaty (NPT), signed in 1968, would be the text that would consolidate it. After the bombing of Hiroshima and Nagasaki in 1945, humanity was conscious of the danger of these weapons, and nuclear proliferation turned into one of the main problems of the Cold War period; their control and the implementation of strategies to limit them have become a priority since then. During the Cold War, nuclear weapons and deterrence policy were crucial elements in the peaceful coexistence of the two power blocs, and the initiatives to control them grew, as both countries were conscious of the danger that this accumulation could cause. The NPT created two categories of states: the “officially” nuclear ones, which could maintain their weapons (China, France, the United Kingdom, the Soviet Union, and the United States) and the nonnuclear ones, which were not allowed to acquire or develop them. Two more concepts emerged: vertical proliferation (that of the five official nuclear states) and horizontal proliferation (that of the states that had nuclear weapons but rejected to be a NPT party). Other treaties—multilateral, regional, and bilateral—which also sought to control the nuclear proliferation (see Treaties and Agreements Preventing Nuclear Weapons Proliferation) were subsequently added. The end of the Cold War did not eliminate the danger. In fact, the Security Council considered in 1992 (Document S/23500, 31 January) that the proliferation of nuclear weapons “constitutes a threat for the international peace and security” (p. 4) that permitted it to activate, if necessary, chapter VII of the United Nations (UN) Charter and all the consequences derived from it. With the new millennium, the United Nations Secretary-General described mass destruction arms (nuclear included) as one of the threats to peace and security in the 21st century (see United Nations General Assembly 2005, cited under Security Council, General Assembly, and Secretary-General, para. 78). Nowadays, the nuclear question is still of great relevance. The nuclear problems in the 21st century’s international society are wide and varied and include states that withdrew the NPT (North Korea), states that fail to comply with it (Iran), states that have not yet ratified it (Israel, India, Pakistan), and non-state actors (such as terrorist groups), which are more and more interested in the wide destructive power of nuclear weapons. The adoption of the Treaty on the Prohibition of Nuclear Weapons of 7 July 2017 was a significant step, but the low number of state accessions shows that nuclear weapons are still a relevant threat.