A good brief summary of Realism approach to foreign policy by Stephen Walt:
To remind you: 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.
What Would a Realist World Have Looked Like?