How do you distinguish a foreign policy “idealist” from a “realist,” an optimist from a pessimist? Ask one question: Do you believe in the arrow of history? Or to put it another way: Are we condemned to do the same damn thing over and over – or is there hope for some enduring progress in the world order?

For realists, generally conservative, history is an endless cycle of clashing power politics. The best we can do is to defend ourselves, managing instability and avoiding catastrophe. But expect no essential alteration in the course of human affairs.

The idealists believe otherwise. They believe that the international system can eventually evolve out of its Hobbesian state of nature into something more humane and hopeful. What is usually overlooked is that this hopefulness for achieving a higher plane of global comity comes in two flavors – one liberal, one conservative.

The liberal variety (as practiced, for example, by the Bill Clinton administration) believes that the creation of a dense web of treaties, agreements, transnational institutions and international organizations (like the U.N., nongovernmental organizations, the World Trade Organization) can give substance to a cohesive community of nations that would, in time, ensure order and stability.

The conservative view (often called “neoconservative,” and dominant in the George W. Bush years) is that the better way to ensure stability is not through international institutions, which are generally powerless, but through the spread of democracy. Because, in the end, democracies are inherently more inclined to live in peace.

Liberal internationalists count on globalization, neoconservatives on democratization to achieve international harmony. But what unites them is the belief that such a state exists and is achievable. Both believe in the perfectibility of the international system. Both believe in the arrow of history.

For realists, it’s a comforting delusion that gives high purpose to international exertions where none exists. Sovereign nations remain in incessant pursuit of power and self-interest. The pursuit can be carried out more or less wisely. But nothing fundamentally changes.

Barack Obama is a classic case study in foreign policy idealism. Indeed, one of his favorite quotations is about the arrow of history: “The arc of the moral universe is long, but it bends toward justice.” He has spent nearly eight years trying to advance that arc of justice. Hence his initial “apology tour,” that burst of confessional soul-searching abroad about America and its sins, from slavery to the loss of our moral compass after 9/11. Friday’s trip to Hiroshima completes the arc.

Unfortunately, with “justice” did not come peace. The policies that followed – appeasing Vladimir Putin, the Iranian mullahs, the butchers of Tiananmen Square and lately the Castros – have yielded nothing but geopolitical chaos and human suffering. (See Syria.)

But while two terms as president may not have disabused Obama of his arc-of-justice idealism (see above: Hiroshima visit), they have forced upon him at least one policy of hardheaded, indeed hardhearted, realism. On his Vietnam trip this week, Obama accepted the reality of an abusive dictatorship while announcing a warming of relations and the lifting of the U.S. arms embargo, thereby enlisting Vietnam as a full partner in the containment of China.

This follows the partial return of the U.S. military to the Philippines, another element of the containment strategy. Indeed, the Trans-Pacific Partnership itself is less about economics than geopolitics, creating a Pacific Rim cordon around China.

Obama thus leaves a double legacy. His arc-of-justice aspirations, whatever their intention, leave behind tragic geopolitical and human wreckage. Yet this belated acquiescence to realpolitik, laying the foundations for a new containment, will be an essential asset in addressing this century’s coming central challenge, the rise of China.

I don’t know – no one knows – if history has an arrow. Which is why a dose of coldhearted realism is always welcome. Especially from Obama.

Charles Krauthammer is a columnist for The Washington Post. He can be contacted at:

letters@charleskrauthammer.com