Stephen Walt, my favorite advocate of the realist viewpoint, explains it briefly in What Would a Realist World Have Looked Like? Foreign Policy 01/08/2016:
Realism sees power as the centerpiece of political life and sees states as primarily concerned with ensuring their own security in a world where there’s no world government to protect them from others. Realists believe military power is essential to preserving a state’s independence and autonomy, but they recognize it is a crude instrument that often produces unintended consequences. Realists believe nationalism and other local identities are powerful and enduring; states are mostly selfish; altruism is rare; trust is hard to come by; and norms and institutions have a limited impact on what powerful states do. In short, realists have a generally pessimistic view of international affairs and are wary of efforts to remake the world according to some ideological blueprint, no matter how appealing it might be in the abstract.He gives several examples of how a failure of realistic thinking has led to bad foreign policy decisions for the US.
Paul Pillar is also someone I often quote here. And his analyses are impressive. He identifies with the "realist" approach. But the examples he gives in The Forgotten Benefits of Offshore Balancing National Interest 01/27/2016 are a reminder of the grim side of foreign-policy "realism":
During the Iran-Iraq War of the 1980s, for example, the United States was officially neutral. When it appeared that Iraq would have difficulty keeping up the fight, the Reagan administration tilted toward Baghdad—a tilt that, in light of history that would unfold two decades later, made for strange bedfellows. Saddam Hussein's role as the original aggressor, his use of chemical weapons, and humanitarian considerations involving the enormous costs of the war to both sides took a back seat to the idea that the United States would not benefit from either side being a clear winner. It would be better from the standpoint of U.S. interests and the prevention of anyone gaining regional hegemony to have both sides suffer from an exhausting stalemate. The idea was valid, although the same U.S. policymakers later mishandled policy toward the war with what became the Iran-Contra scandal. [my emphasis]James Carden also discusses the extent to which the "realist" perspective has been sidelined in practice in official circles in How Neocons Banished Realism Consortium News 01/16/2016.
He notes, as Walt also indicates, that the "liberal interventionists" have become largely indistinguishable from the neocons in practice.