In the context of arms control, what constitutes 'horizontal proliferation'?

In the context of arms control, 'horizontal proliferation' refers to the spread of weapons of mass destruction (WMD), particularly nuclear weapons, to countries that do not already possess them. It signifies the acquisition of these weapons by new states, increasing the number of actors capable of deploying them. This contrasts with 'vertical proliferation,' which refers to the development and accumulation of more weapons or more advanced weapons technologies by states that already possess them. Horizontal proliferation is a primary concern in arms control because it increases the risk of WMD use, whether through deliberate action, miscalculation, or accident. The more countries that possess these weapons, the greater the chance that they will be used in a conflict, potentially leading to catastrophic consequences. The Nuclear Non-Proliferation Treaty (NPT) is a key international agreement aimed at preventing horizontal proliferation by committing non-nuclear weapon states not to acquire nuclear weapons and requiring nuclear weapon states to pursue disarmament.