Not long after the last world war, the historian William L. Shirer had this to say about the next world war. It “will be launched by suicidal little madmen pressing an electronic button. Such a war will not last long and none will ever follow it. There will be no conquers and no conquests, but only the charred bones of the dead on an uninhabited planet.”

As an investigative journalist, I write about war, weapons, national security and government secrets. I’ve previously written six books about US military and intelligence programmes – at the CIA, The Pentagon, Defense Advanced Research Projects Agency– all designed to prevent, or deter, nuclear world war III. In the course of my work, countless people in the upper echelons of US government have told me, proudly, that they’ve dedicated their lives to making sure the US never has a nuclear war. But what if it did?

“Every capability in the [Department of Defense] is underpinned by the fact that strategic deterrence will hold,” US Strategic Command (STRATCOM), which is responsible for nuclear deterrence, publicly. Until the autumn of 2022, this promise was pinned on STRATCOM’s public Twitter feed. But to a private audience at Sandia National Laboratories later that same year, STRATCOM’s Thomas Bussiere the existential danger inherent to deterrence. “Everything unravels itself if those things are not true.”

If deterrence fails – what exactly would that unravelling look like? To write , I put this question to scores of former nuclear command and control authorities. To the military and civilian experts who’ve built the weapon systems, been privy to the response plans and been responsible for advising the US president on nuclear counterstrike decisions should they have to be made. What I learned terrified me. Here are just a few of the shocking truths about nuclear war.

The US maintains a nuclear launch policy called Launch on Warning. This means that if a military satellite indicates the nation is under nuclear attack and a second early-warning radar confirms that information, the president launches nuclear missiles in response. Former secretary of defense William Perry told me: “Once we are warned of a nuclear attack, we prepare to launch. This is policy. We do not wait.”

The US president has sole authority to launch nuclear weapons. He asks permission of no one. Not the secretary of defense, not the chairman of the joint chief of staff, not the US Congress. “The authority is inherent in his role as commander in chief,” the Congressional Research Service confirms. The president “does not need the concurrence of either his [or her] military advisors or the US Congress to order the launch of nuclear weapons”.

When the president learns he must respond to a nuclear attack, he has just 6 minutes to do so. Six minutes is an irrational amount of time to “decide whether to release Armageddon”, President Ronald Reagan lamented in his memoirs. “Six minutes to decide how to respond to a blip on a radar scope… How could anyone apply reason at a time like that?” And yet, the president must respond. This is because it takes roughly just 30 minutes for an intercontinental ballistic missile to get from a launch pad in Russia, North Korea or China to any city in the US, and vice versa. Nuclear-armed submarines can cut that launch-to-target time to 10 minutes, or less.

Today, there are nine nuclear powers, with a combined total of more than 12,500 nuclear weapons ready to be used. The US and Russia each have some 1700 nuclear weapons deployed – weapons that can be launched in seconds or minutes after their respective president gives the command. This is what Shirer meant when he said: “Such a war will not last long and none will ever follow it.”

Nuclear war is the only scenario other than an asteroid strike that could end civilisation in a matter of hours. The soot from burning cities and forests will blot out the sun and cause nuclear winter. Agriculture will fail. Some 5 billion people will die. In the words of former Soviet leader Nikita Khrushchev, “the survivors will envy the dead”.

I wrote Nuclear War: A scenario to demonstrate – in appalling, minute-by-minute detail – just how horrifying a nuclear war would be. “Humanity is one misunderstanding, one miscalculation away from nuclear annihilation,” UN secretary-general António Guterres warned the world in 2022. “This is madness. We must reverse course.”

How true.

��by Annie Jacobsen, published by Torva (£20.00), is available now. It is the latest pick for the 91av Book Club: sign up��here��to read along with our members