Nuclear weapons are back on the agenda. There is now a greater risk of them being used than at any time since the 1960s. While we wrestle with America’s global retreat, Brexit, and dealing with Islamist terrorism, we must not lose sight of the one issue that could upend the international order and destroy our way of life.

There are now nine states with nuclear weapons. For six of them — the US, Britain, France, Israel, India and China — they are purely defensive weapons, designed solely as the ultimate means to deter attack. The remaining three think differently. Russia and Pakistan also conceive of using nuclear weapons as a means of turning a limited conflict in their favour. North Korea wants nuclear weapons to hold others at threat, both to protect the regime and to secure more practical benefits.

Nuclear weapons create a military balance where one does not exist between conventional forces. During the cold war, the Soviets had superior armed forces and Nato had to rely on the threat of nuclear retaliation to keep the peace. The balance is now reversed. However skilful Russia’s use of the new weapons of hybrid warfare, the balance between regular forces is now reversed. Russia’s military doctrine also provides for battlefield nuclear weapons being used to bring a war in central Europe to an end on Russia’s terms. Its forces train for that scenario, and we have to take it seriously.