Key point: The British and American alliance with Russia only lasted as long as Hitler.

In the spring of 1945, Winston Churchill asked his military chiefs to prepare a secret plan.

That was nothing new. The hyper-energetic Churchill was always coming up with plans, some clever and some crazy. But this plan was beyond all that.

Winston Churchill wanted a plan for Britain to invade the Soviet Union.

In early 1945, America was focused on finishing off Germany and then taking down Japan. But Churchill's gaze beheld a darkness descending upon Europe. What would happen with a Red Army occupying its heart? Stalin had already reneged on earlier agreements that Poland—the reason that Britain had gone to war in 1939—would be free. Instead the Polish government was packed with Soviet supporters while Polish resistance fighters ended up in NKVD prisons. Romania, Hungary and Czechoslovakia were under Soviet control, and Greece and Turkey appeared under threat. After Germany's inevitable surrender, the huge U.S. force in Europe would move to the Pacific.

Who would be left to stop the Russians?

Thus British planners devised “Operation Unthinkable,” an apt name for what would have been World War III. What could be a more unimaginable task then trying to devise some way for Britain—broke and exhausted after two world wars—from launching a preventive war to defeat the Soviet colossus?

Yet even if Great Britain was losing the “Great” by 1945, orders were orders, and military planners are accustomed to devising responses to the most unlikely contingencies. So they gamely went to work, and by 1945 had worked out a plan. The attack would begin on July 1, 1945, to allow operations before the winter weather arrived. They assumed that Soviet intelligence would detect Allied preparations and thus make an Operation Barbarossa–style surprise offensive impossible. Thus the Allies would have a tough fight right from the start.

What a War Between America and China Would Look Like. 

Operation Unthinkable envisioned an offensive by the Anglo-American armies, plus a Free Polish contingent (the Canadians were also informed of the plan). These forces would breach the forward Soviet defenses in Germany. The expectation was that the Soviets would then mass their armor along the Oder and Neisse rivers, which the Soviets had made the new border between Germany and Poland. A gigantic Kursk-like armored battle would be fought around Stettin. If the Allies won it, they would advance to a 250-mile-long line between Danzig and Breslau, where they would halt to avoid exposing their flank to a southern attack from Soviet forces in Czechoslovakia.

Ironically, the plan bore many resemblances to Hitler’s Operation Barbarossa, which also counted on defeating Soviet forces near the Russian border to avoid a prolonged campaign deep inside that vast nation. “The planners believed that if they could secure this line from Danzig to Breslau by autumn 1945, it might be enough to bring Stalin to heel,” writes author Jonathan Walker in his book Churchill's Third World War: British Plans to Attack the Soviet Empire, 1945. “But if the Allies reached that line by the autumn (discounting the huge advantage the Soviets held in manpower) and Stalin had not changed his mind about control of Eastern Europe—what then? With the forces available to them, Western commanders could not hold their line through the winter of 1945–46 and they would be forced either to retreat or push on into eastern Poland and the Soviet Union. Pushing on would undoubtedly result in ‘total war.’”

Total war against Russia—months before the first atomic bomb was dropped on Japan—was an outcome that no one wanted. The Allied forces had nearly 4 million men in Europe when Germany surrendered, the majority of which were Americans who would soon be transferred to the Pacific. The Red Army had almost 11 million men, and perhaps 20,000 tanks and self-propelled guns. To be sure, the Allies did count on the same advantages that enabled them to defeat Nazi Germany. They had vast superiority at sea, which meant their fleets could provide amphibious support in the Baltic Sea. The Allied tactical air forces would be outnumbered two to one by Soviet tactical air, but the Allies could count on better-trained pilots and the fact that the Soviets depended on the United States for high-octane aviation fuel. However, the real ace in the air would be the 2,500 Allied heavy bombers in Europe, which presumably would include B-29s. The Luftwaffe hadn’t been able to stop them, and the Red Air Force had no experience in stopping them.

Nonetheless, the Allied planners found themselves in the same trap that destroyed Napoleon and Hitler. How do you make Russia surrender if it doesn't want to? If defeating the Red Army on German soil wasn't enough, then the only alternative was to advance eastwards into Poland and then Russia. “The planners now paled at the thought of the enormous distances the Allies would have to penetrate to secure victory,” Walker notes.

Meanwhile, the Allies had to reckon on the war expanding as the Soviets attacked Norway, Greece and Turkey (ominously, British planners expected the Soviets to ally with Japan). As for the atom bomb, the United States only had two in the summer of 1945, and they were earmarked for Japan. By 1946, America had only nine bombs. Powerful as they were, they could only inflict a fraction of the punishment that the Soviet Union suffered at the hands of the Nazis—and still kept on fighting.

What’s fascinating isn't just the hubris—or chutzpah—of Britain invading Russia, something which it hadn't done since the Crimean War. It's the assumptions behind the plan, driven either by wishful thinking or sheer desperation.

Even as the death camps were being liberated, Britain contemplated rebuilding a German army to fight the Russians. “One of the most contentious issues in the Unthinkable plan was the use of German forces within the Allied camp,” Walker writes. “It was anticipated that ten German divisions could be utilized for offensive operations, but because it would take time for them to be re-equipped from Allied sources, the units would not be ready for 1 July and would only become available in the autumn; that they should be used at all was likely to be highly controversial.”

But rearming ex-Nazis paled in comparison to an absolute foundation of Operation Unthinkable, which was that the United States would join Britain in attack on the Soviet Union. Roosevelt, and initially Truman until he knew better, were convinced that it was possible to work out a postwar accommodation with Stalin. They were wrong, but they didn't know that in the spring of 1945. And there was still the victory with Japan to be won—for which Soviet help was considered essential. In other words, America had just finished a crusade in Europe against Nazism. It wasn't about to embark on a crusade against Communism just yet.

Military buffs love to debate how a war between the Western Allies and Soviets would have turned out (though the assumption is usually that the Soviets would have attacked first). Enthusiasts love to argue the merits of Sherman vs. T-34 tanks, or P-51s versus Yak fighters. It's all very interesting, and almost totally pointless.

The rock-bottom fact of a war that would have dragged the world into World War III is this: Operation Unthinkable called for the democratic nations of the United Kingdom and the United States to initiate a war with the Soviet Union. The justification would have been the need to roll back the Soviet empire from its German and Eastern European conquests.

In return, the populations of Britain and America would be expected to endure a protracted conflict with no certain means of compelling the enemy to surrender. Rather than the relatively bloodless air and naval warfare that the Anglo-Americans preferred and still prefer, they would have been trapped in a land war with the world's foremost land power, on the vast, cold plains and swamps of Eastern Europe.

Operation Unthinkable was truly unthinkable.

Further reading: Operation Unthinkable: The Third World War: British Plans to Attack the Soviet Empire 1945.

Michael Peck is a contributing writer for the National Interest. He can be found on Twitter and Facebook. This article first appeared several years ago.

Image: Reuters.