The 2011 TIME 100

"We remain a young nation," Barack Obama said in 2009, but he added an unsettling admonition that "in the words of Scripture, the time has come to set aside childish things." No passage in his Inaugural Address more vividly reflected the President's vision of his country and his times or more accurately foreshadowed the vexations that were to beset his leadership.

Like FDR before him, Obama, 49, has looked beyond the near horizon. He has paid the political price of setting far-visioned initiatives on health care and financial reform ahead of short-term relief. And he has tried to persuade his countrymen to shed some of their youthful illusions: to forsake the frontiersman's faith in unbridled individualism for a recognition of the complex interdependencies of modern life, to replace the rebel's fear of government with the citizen's trust that government of the people and by the people is for the people too, to stop assuming that Santa Claus will give us cheap energy forever and the Easter Bunny will pay our bills. Whatever the near term holds, history is likely to record that Obama set the country on the path to a future with fewer illusions.

Kennedy is a professor of history at Stanford University