Do leaders really matter? That question has prompted debate for centuries. The Scottish philosopher Thomas Carlyle famously represented one side of the argument. "The history of the world is but the biography of great men," he wrote. Leo Tolstoy spoke for the other side. Great men, the novelist wrote, "are but the labels that serve to give a name to an end and, like labels, they have the least possible connection with the event."

But surely the 20th century resolved the issue that individual leaders do matter--a lot. Consider: As the 1900s opened, hopes ran high that the new century would bring a golden age, perhaps rivaling the Renaissance. European nations had not engaged in a general war for more than 80 years, trade spurred growth, and new discoveries were transforming life.

Yet the first half of the century brought the bloodiest wars in all of human experience. The international economy fell into darkness, and the number of democracies dwindled.

What happened? One of the most persuasive answers comes from the British historian John Keegan. The political history of the 20th century, he wrote, can be found in the biographies of six men: Lenin, Stalin, Hitler, Mao Zedong, Churchill, and Franklin Roosevelt. The first four were tyrants who almost drove the world over a cliff; had it not been for the final two, western civilization might have perished.

Historian Arthur Schlesinger Jr. poses this question. In 1931, a British politician visiting New York City was struck by a car and nearly squashed. Fourteen months later, an American politician was sitting in an open car in Miami when a gunman opened fire and--had a woman not jarred his arm--would have killed his target. How would history have been different, asks Schlesinger, if Winston Churchill and Franklin Roosevelt had died?

Yes, the quality of leaders matters, and what we have learned is that it matters across American life. General Electric would not have flourished as it did had Jack Welch not been at the helm. Martin Luther King Jr. inspired with his dream; Eleanor Roosevelt opened doors for millions closed out; Rachel Carson sparked the environmental movement. Would we have gotten to the moon without John Kennedy?

The issue today is whether America can nourish enough good leaders to forge a bright path into the 21st century. There are, after all, some ominous parallels to the early 20th century. As we opened this new century, Americans stood as the most powerful people since ancient Rome. Peace and rising prosperity were within our grasp; globalization and technology offered grand vistas. But recent years have brought sharp reversals. Now we worry that caldrons of terrorism are boiling over, that our culture is declining, that our politics are dysfunctional, that our best jobs may disappear to China and India.

"Perhaps no form of government needs great leaders so much as democracy," Lord Bryce observed. Nowhere is that more true than in America today.

No spotlight. What emerges from this initial Best Leaders project is that America has many worthy leaders, especially in business and the nonprofit worlds. And increasingly the two worlds intersect. Bill Gates, for instance, is best known for Microsoft and personal fortune, but he and his wife, Melinda, are now leaders of social change.