But after Vietnam, most liberals moved on. It became unfashionable to talk about evil. Some liberals came to believe in the inherent goodness of man and the limitless possibilities of negotiation. Some blamed conflicts on weapons systems and pursued arms control. Some based their foreign-policy thinking on being against whatever George W. Bush was for. If Bush was an idealistic nation-builder, they became Nixonian realists.

Barack Obama never bought into these shifts. In the past few weeks, he has revived the Christian realism that undergirded cold war liberal thinking and tried to apply it to a different world.

Obama’s race probably played a role here. As a young thoughtful black man, he would have become familiar with prophetic Christianity and the human tendency toward corruption; familiar with the tragic sensibility of Lincoln’s second inaugural; familiar with the guarded pessimism of Niebuhr, who had such a profound influence on the Rev. Dr. Martin Luther King Jr.

In 2002, Obama spoke against the Iraq war, but from the vantage point of a cold war liberal. He said he was not against war per se, just this one, and he was booed by the crowd. In 2007, he spoke about the way Niebuhr formed his thinking: “I take away the compelling idea that there’s serious evil in the world and hardship and pain. And we should be humble and modest in our belief we can eliminate those things. But we shouldn’t use that as an excuse for cynicism and inaction.”

His speeches at West Point and Oslo this year are pitch-perfect explications of the liberal internationalist approach. Other Democrats talk tough in a secular way, but Obama’s speeches were thoroughly theological. He talked about the “core struggle of human nature” between love and evil.

More than usual, he talked about the high ideals of the human rights activists and America’s history as a vehicle for democracy, prosperity and human rights. He talked about America’s “strategic interest in binding ourselves to certain rules of conduct.” Most of all, he talked about the paradox at the core of cold war liberalism, of the need to balance “two seemingly irreconcilable truths”  that war is both folly and necessary.

He talked about the need to balance the moral obligation to champion freedom while not getting swept up in self-destructive fervor.

Obama has not always gotten this balance right. He misjudged the emotional moment when Iranians were marching in Tehran. But his doctrine is becoming clear. The Oslo speech was the most profound of his presidency, and maybe his life.