History isn't just what's happened. It's also what happened in the context of what might have occurred. This isn't counterfactual history -- it's a way to illustrate what was at stake. And although largely unknown in the West, much was at stake during the Polish-Soviet War. The conflict determined whether Poland would remain free and whether Europe would be exposed to Bolshevik revolution. The conflict was a potential turning point, and one that could have a profound impact on subsequent events.

The Polish-Soviet War started in February 1919. The Russians had an army of more than 5 million men and had assembled 70 divisions for Operation Vistula, while the Polish Army, with less than a million men, could field only 20 divisions to stop them. The war was perhaps the last of its kind, involving a rich mixture of cavalry and tanks, lance and machine gun, of hardened professionals and untested civilians.

By the summer of 1920, the Red Army was approaching Warsaw, and both sides knew that the ensuing battle would determine the outcome of the war. But it was more than a territorial squabble. It was a clash of ideologies: Christianity vs. atheism, individual liberty vs. state control. Lenin believed that by destroying Poland, he would create a Red Bridge to Europe -- particularly Germany -- which he was certain was ripe for Communist revolution.

Despite the significance of the battle, Poland stood alone. Western powers sent diplomats and advisors, not troops, and Bolshevik agitators in western countries organized a near total boycott of military supplies.

The Polish army, isolated and outnumbered, was led by an unlikely hero named Joseph Pilsudski. Having no formal training, many considered him a military lightweight. Both the Russians and the Western powers disliked him, as he had shown an unnerving tendency to place Polish interests above all others. The West knew little about Pilsudski, and what they knew disturbed them. He was described as a former Socialist, a terrorist, a train-robber, and a prison escapee that, through a series of unlikely events, had emerged as leader of the Polish state. They were largely unaware of his decades-long struggle for Polish independence or of the keen intellect that guided his policies.

Unlike his professional western advisors, Pilsudski knew Polish forces were too small for a protracted defense, as it would only invite a war of attrition that the Soviets would welcome. He knew the situation required a bold stroke that could not only defeat the Russians, but also do it quickly, before they could utilize their endless pool of manpower. In a move his advisors said broke all the rules, Pilsudski removed men from the Warsaw defenses and assembled them in secret, planning to attack the Red Army at a right angle. If his unorthodox venture failed, Poland, which had regained her freedom only two years before after over a century of slavery, would again be lost.

Somehow -- some would say miraculously -- Pilsudski prevailed and the Bolsheviks were caught completely off guard. The Polish victory at the Battle of Warsaw changed the strategic picture so abruptly that it is difficult to think of another comparable operation in the annals of European military history. Total defeat became total victory in the blink of an eye, based on the actions of a small group of weary soldiers led by an amateur.

As a result of the Miracle on the Vistula, Polish independence was preserved and the Bolshevik Revolution was stopped at the Polish border. The 1919-21 Polish-Soviet War should be considered one of the most consequential conflicts in history. Despite its significance, the war has never received lavish historic attention, perhaps because it is commonly viewed from a counterfactual perspective. It was a historical turning point that refused to turn, significant only because it prevented or delayed what might have happened.

But if history had turned, Europe could have been radically altered. If the Red Army had entered war-torn, revolution-prone Germany in the aftermath of the Great War -- an event requiring no fantastic assumptions -- a Soviet dictatorship may well have spread to the Atlantic shore. Of course, it will never be known if communism could have swept through Western Europe if Russia had defeated Poland. The Soviets later claimed that their crusade failed to ignite a general communist uprising because the Red Army was never able to penetrate into large industrial areas, where workers would have welcomed socialist revolution.

This may be true, but it overlooks a broader point. When the Soviets appeared to be an unstoppable force of history, spearheaded by an irresistible people's army, many in the West were infatuated with communism as the wave of the future. By inflicting a clear-cut, overwhelming military defeat on the Red Army, the Poles not only prevented the Soviets from physically invading Europe, but destroyed their aura of invincibility, and hence, the intoxicating appeal of inevitability.

Pilsudski's actions preserved the Versailles Peace for another two decades, but the task would have been much easier, and the effect more lasting, if the Western democracies had stood with Poland. Although most Americans are unaware of this history, it is important to understand that events in what we consider obscure outposts can have far-reaching implications. Rather than ignore struggles against tyranny in foreign lands, American policy is best served by supporting people who value freedom, wherever they may be.

Peter Hetherington is the author of Unvanquished: Joseph Pilsudski, Resurrected Poland and the Struggle for Eastern Europe, winner of the 2012 Independent Book Publishers Association's Ben Franklin Award in history. Unvanquished ($18) is available at amazon.com.