In early December 1980, Warsaw Pact infantry and armored divisions, most of them Soviet, moved into position along Poland's borders. Three months earlier, the Solidarity movement had sprung to life in Gdansk's Lenin Shipyards. After a tumultuous infancy, Solidarity seemed doomed to be strangled in its cradle, as Czechoslovakia's "Prague Spring" had been in 1968.

The stakes were enormously high. A Warsaw Pact invasion of Poland would almost certainly have met with violent resistance from the aroused Poles. Had massacres ensued, would the West have remained idle? As the calendar pages turned in the first week of December 1980, it seemed entirely possible that Poland, the flashpoint that ignited World War II in Europe, would be the flashpoint that ignited World War III --- a war that could have been fought with nuclear weapons.

Then, remarkably, there was no invasion. Warsaw Pact troops stopped advancing toward Poland and then retreated. What had happened?

While I was preparing Pope John Paul II's biography, I talked about that hair-raising period with one of the wisest men I've ever known, Jan Nowak, former director of Radio Free Europe's Polish service (and a recipient of the Presidential Medal of Freedom, America's highest civilian honor). Jan told me that two men had saved Poland from invasion.

One was a familiar name: Zbigniew Brzezinski, who in December 1980 was serving his last weeks as President Carter's national security adviser. As Jan unfolded the tale, though, Brzezinski could do what he did because of an unheralded figure who died in relative obscurity last month, after living in exile under an assumed name for 23 years. His real name was Ryszard Kuklinski.

By 1980, Colonel Kuklinski had spent his entire adult life in the Polish Army. A tour with the International Control Commission in Vietnam in 1967-68, where he met many Americans, convinced him that communist propagandists were painting a false portrait of the United States.

Then, in August 1968, the reform communism of the "Prague Spring" was crushed beneath the treads of Soviet tanks; Kuklinski was appalled. His concerns increased exponentially when he became a senior Polish Army war-planner with access to the highest-level information. In his new position, Kuklinski learned that Soviet military doctrine anticipated a western nuclear response to a Soviet invasion of western Europe --- a response that would fall, not on the USSR (which would risk global catastrophe) but on Poland, as the second wave of Soviet troops, tanks, and materiel passed through Kuklinski's homeland en route to the west. The USSR, Kuklinksi concluded, was no "fraternal ally;" it was a predator, prepared to sacrifice Poland for its own aggressive purposes.

What was a Polish patriot to do? Kuklinksi offered his services to the United States and for nine years (1972-1981) was the single most important western intelligence asset behind the iron curtain. At daily risk of his life, Colonel Kuklinski provided the U.S. government with some 50,000 pages of highly-classified documents that were of immeasurable assistance to Western defense planners and arms control negotiators.

Ryszard Kuklinski's greatest service came in the Solidarity crisis of late 1980, when he gave the U.S. the entire operational plan for the proposed Warsaw Pact invasion of his homeland. With that in hand, Zbigniew Brzezinski and private-sector leaders like the AFL-CIO's Lane Kirkland were able to organize an international trade embargo of the USSR, should an invasion take place; such an embargo, a de facto blockade, would have been a devastating blow to the already tottering Soviet economy, and the Soviets backed off.

Martial law, imposed in Poland a year later, was bad enough. A Warsaw Pact invasion, given its possible international consequences, risked Armageddon. By providing the crucial intelligence that helped forestall the invasion, Colonel Kuklinski may well have prevented a nuclear holocaust.

Kuklinski's remarkable story is now told in gripping detail in Benjamin Weiser's A Secret Life: The Polish Officer, His Covert Mission, and the Price He Paid to Save His Country (Public Affairs). I wish the book a broad readership. Despite the fact that Ryszard Kuklinski was abruptly taken from us by death last month, it's never too late to get to know a man of principle, a true hero of freedom.

George Weigel is a senior fellow of the Ethics and Public Policy Center in Washington, D.C.