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.