David Goldman applies what he calls “Autustinian realism” to America’s foreign affairs and comes up with a variety of interesting conclusions:

What we might call “Augustinian realism” is this premise, borne out in the world around us. To the extent that other nations share the American love for the sanctity of the individual, they are likely to succeed. To the extent they reject it, they are likely to fail. Our actions in the world can proceed from American interest–precisely because American interest consists of allying with success and containing failure.
Augustinian realism begins with the observation that civil society precedes the character of a nation. The American state can ally with, cajole, or even crush other states, but it cannot change the character of their civil society, except in a very slow, gradual, and indirect fashion–for example, through the more than 100,000 American Christian missionaries now working overseas. This realism insists that the state should not try to do what it cannot do.

For the most part, he finds the Bush administration’s policies unrealistic, but Obama’s “baffling”:

Instead of a president determined to use American hegemony to rid the world of evil, America has a president determined to rid the world of hegemony. As Barack Obama told the United Nations last September, “No one nation can or should try to dominate another nation. No world order that elevates one nation or group of people over another will succeed. No balance of power among nations will hold.” Since America is the only nation capable of exercising hegemony on a world scale or maintaining the balance of power among other powers, President Obama’s doctrine is the self-liquidation of American influence–an unprecedented and, on reflection, astonishing position for an American leader.

In Goldman’s view, the United States is morally obliged to ally with and help other nations (and out-of-power factions) that share our understanding of civic society. That obligation is tempered, however, by a realistic acceptance that we cannot change other cultures in the same way that we can topple regimes and build schools.
It doesn’t make for easy calculations, but in our messy world, no coherent political philosophy would. Prudential decisions must be made to remove threats, as I would argue Saddam Hussein represented and as most agree the Taliban did, but as Goldman argues, becoming “entangled in unrealistic objectives” has made our military a sort of hostage for the threats of Iran as it races toward nuclear weapons. But one could move on from there to lament years of mishandling Iran.
The difficulty that democracy presents is that foreign policy that is consistent over time requires a certain amount of cultural consistency among voters, and our culture has gained a distinct wobble over the past half-century.