Ever since people began to speculate about “World War III,” its very name has implied its own inevitability.

The phrase seems to have emerged during the early 1940s, not long after people began to think of the “Great War” of 1914 as World War I—a precursor to World War II. Time magazine mused about a third war as early as November 1941, one month before the attack on Pearl Harbor and the U.S. entry into World War II. More consequentially, British Prime Minister Winston Churchill started planning for a World War III while he was still fighting the second war. And he kept on worrying about it, too.

“After he’s out office, during the summer of 1945 through 1946, he continually believes that the Soviet Union are going to start another war,” says Jonathan Walker, a military history writer and author of Operation Unthinkable_: The Third World War_.

“He always realized that there would be an eventual threat,” Walker says. But “the attitude of Britain and the U.S. were quite out of kilter in what they believed the Soviet Union was capable of,” and Churchill struggled to convince U.S. President Franklin D. Roosevelt of his point of view.

This doesn’t mean that no one in the U.S. thought a future World War III might involve the Soviet Union. In 1943, Roosevelt’s second vice president, Henry A. Wallace, predicted that “unless the Western democracies and Russia come to a satisfactory understanding before the war ends, I very much fear that World War III will be inevitable” (this quote comes from one of Time’s many articles from the 1940s that speculated about World War III).

But Churchill’s concern was on another level—which is why he created a secret (and later scrapped) military plan to invade Soviet-occupied territory.

“Operation Unthinkable” called for British, American, Polish and German troops to invade Soviet-occupied East Germany and Poland on July 1, 1945. At that point, Germany had surrendered, but the war in Asia wasn’t over yet. The goal of the plan was to drive the Soviets out and secure those parts of Eastern Europe.

But in the end, Churchill’s military advisors convinced him that the plan was too risky. FDR died in April of 1945, less than a month before the war in Europe ended, and by July, Churchill had lost his seat as prime minister.

“It’s not until the summer of 1946 that British and U.S. military planners finally get together to work out a coordinated plan in the event of a World War III,” Walker says.

The plan, “Operation Pincher,” called for atomic airstrikes and ground invasions of the Soviet Union. But as the realities of the United States’ new nuclear capabilities settled in, and as the concern with Soviet nuclear developments grew, the specter of a third war took on new, terrifying possibilities in the public imagination.

The introduction of the atomic bomb in the 1940s and the hydrogen bomb in the 1950s gave the phrase “World War III” a new, specific meaning: nuclear annihilation. And as the U.S. and the Soviet Union became locked in a Cold War, it seemed that nuclear war could break out at any moment.

Writers had already been stoking people’s fears about nuclear energy long before the U.S. harnessed it into bombs, says Spencer R. Weart, a science historian and author of The Rise of Nuclear Fear. When scientists first discovered radioactivity and nuclear energy at the turn of the century, it evoked both awe and terror. One of them, Frederick Soddy, thought “it might even be possible to set off an explosion that would destroy the entire world,” Weart says.

Some writers translated this horrifying possibility in fiction. In his 1914 book The World Set Free, H.G. Wells coined the term “atomic bomb” three decades before the first successful nuclear bomb test. And in the 1938 novel The Doomsday Men, a mad scientist declared he would set off a reaction that “would peel the skin off the Earth ‘like an orange, only faster,’” Weart says.

By the early 1940s, the fear that Nazi Germany was developing a nuclear bomb drove the Manhattan Project to develop one first, Weart explains. In actuality, the Nazis were not working on their own nuclear weapon. But after the U.S. developed and used the bomb, the nuclear fear shifted from Germany to the Soviet Union—a country that developed, as new President Harry S. Truman feared it would, a nuclear bomb in 1949.

This is where the threads of Churchill’s fears of Soviet aggression and the existential threat of nuclear war began to intersect. When Churchill became prime minister again in 1951, he continued worrying about the Soviet Union. By then, however, the reality of nuclear annihilation had made invasion-based strategies like “Operation Unthinkable,” and even “Operation Pincher,” irrelevant.

“In the period of Operation Unthinkable,” Walker explains, “they didn’t believe at that stage that [nuclear bombs] are winners of wars. They believed they’re supplementary.” By the time Churchill was prime minister again, that was clearly not the case. Nuclear bombs had the power to destroy more than was ever possible before.

The amassing of nuclear weapons ushered in a new, more destructive kind of World War III to worry about. As the U.S. expanded its nuclear arsenal and developed missile defense shields, a theory of “mutually assured destruction” posited that if either the United States or the Soviet Union used nukes, both would annihilate each other.

Therefore, nuclear bombs began to be seen as a means of deterring others from attacking rather than as weapons to be deployed.

Even though no country has used nuclear bombs as weapons since the U.S. attacked Hiroshima and Nagasaki in 1945, the threat of nuclear war remains. In September 2017, inventor Elon Musk tweeted his concern that artificial intelligence mixed with nuclear weapons—a combination that doesn’t yet exist—could lead to World War III. A month later, Senator Bob Corker (R-Tennessee) told The New York Times that President Donald Trump’s threats to other countries could put the U.S. “on the path to World War III.”