Mutual assured destruction refers to the concept that two superpowers are capable of annihilating each other with nuclear weapons, regardless of whether they are attacked first.

In theory, under mutual assured destruction, a nuclear attack by one superpower will be met with an overwhelming nuclear counterattack by their target — using early warning systems, automated missiles, airborne nuclear bombs, and missile-armed hidden submarines. This will lead to the complete destruction of both. As such, mutual assured destruction — often abbreviated as MAD — is part of the military strategy of deterrence, in which one adversary threatens another with a reprisal if they attack first.

After the 1960s, mutual assured destruction was the main nuclear doctrine — the stated military principle — of the Cold War between the United States and the Soviet Union in the late 20th century. It is still in operation today between the nuclear forces of the U.S. and Russia, and experts have suggested that MAD is the reason that small states such as Israel are thought to have developed nuclear missiles. (Israel is deliberately ambiguous about whether it has them.)

According to a 2007 study in the journal Asian Affairs: An American Review, China, the third nuclear superpower, does not have the capacity to threaten true mutually assured destruction because its relatively small arsenal of nuclear missiles does not have a credible "second strike" capability, which would be needed to automatically respond to a nuclear attack.

Related: The 9 most powerful nuclear explosions

But complete annihilation of an enemy is not the only way MAD comes into play. For instance, "rogue states," like North Korea and Iran, are striving to develop nuclear missiles, perhaps in the hope that they will be able to at least inflict severe damage on an enemy before they are annihilated by a nuclear counterstrike — a partial application of the MAD doctrine, according to a 2019 analysis by the U.S. Department of Defense.

Assured destruction

The term "assured destruction" was first used in the 1960s by then-U.S. Secretary of Defense Robert McNamara, who served in the Kennedy and Johnson administrations. But according to Britannica, the longer phrase "mutual assured destruction" was coined by an opponent of the policy, American military analyst Donald Brennan, who argued that it did little to secure U.S. defense interests in the long-term.

McNamara estimated that a nuclear strike force with the equivalent explosive power of 400 megatons of TNT — a "few hundred" missiles, as some military planners said — was needed to ensure an effective nuclear deterrence, according to the Brookings Institution.

But that MAD number rapidly increased, and by the time of the Carter administration in 1977, military planners argued that the U.S. needed 2,000 nuclear warheads. But about the same time officials said the U.S. needed to reduce its nuclear arsenal to 5,000 warheads, and in the mid-1990s officials talked of reducing the number again to 2,500, so clearly many more had already been built. (These numbers far outstrip the number needed to do catastrophic damage to the planet. For instance, a 2012 study by the Bulletin of the Atomic Scientists found that just 100 nuclear detonations of the size that struck Hiroshima and Nagasaki would usher in a planetary nuclear winter, which would drop temperatures lower than they were in the Little Ice Age, Live Science previously reported.)

Although no one has tested the concept of mutual assured destruction by nuclear weapons, it seems to have prevented war between superpowers since nuclear weapons were invented in the 1940s. But it also led to periods of the Cold War when both the U.S. and the Soviet Union spent huge amounts of money to develop nuclear weapons and the methods to use them.

Experts in nuclear weapons arsenals estimate that the United States had more than 30,000 nuclear warheads in service at the height of the Cold War in the 1960s and 1970s, while the Soviet Union may have stockpiled more than 40,000 warheads by the late 1980s, according to BBC News.

End to war

The concept of mutual assured destruction is even older than nuclear weapons. In the 19th century, the writers Wilkie Collins and Jules Verne both speculated that the industrialization of warfare would make armies so powerful that the countries that fielded them would be locked in a perpetual stalemate.

And several inventors — including Richard Gatling, the inventor of the Gatling gun; Alfred Nobel, the inventor of dynamite; and Nikola Tesla, who hoped to develop particle beam weapons — suggested their weapons would make annihilation of each side inevitable and put an end to war as a consequence.

The concept of mutual assured destruction even made it to the movies. Two popular films released in 1964, at the height of Cold War tensions between the U.S. and the Soviet Union, both dealt with the destruction that could accidently be wrought by the massive nuclear arsenals on standby to wipe each other out: Stanley Kubrick’s "Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb," and "Fail Safe," a thriller directed by Sidney Lumet. Both movies ended in nuclear annihilation, although only for Moscow and New York in “Fail Safe.”

RELATED STORIES

—How many nuclear weapons exist?

—End of the world? Top 10 doomsday threats

—Why it's hard to make nuclear weapons

Since the end of the Cold War, the superpowers have taken steps to limit their nuclear arsenals. By 2004, a report for the Strategic Studies Institute of the U.S. Army War College had declared,"nuclear Mutual Assured Destruction thinking appears to be in decline," with the U.S. planning to develop more accurate nuclear weapons that would reduce the number of civilians killed in a nuclear strike.

But the threat of nuclear annihilation remains real. The Federation of American Scientists, a nonprofit founded in 1945 by scientists and engineers who had worked on the Manhattan Project to develop the first nuclear bomb, reports that as of early 2022, about 12,700 nuclear warheads are possessed today by nine countries: the United States, Russia, the United Kingdom, France, China, India, Pakistan, Israel and North Korea. Most of them are held by the United States and Russia, which have about 4,000 warheads each. And according to a 2018 scientific study in the journal Safety, that's enough to wipe out almost all of us.

Originally published on Live Science.