Realism
Realism is a descriptive and prescriptive theory for international relations.
Formulation
The earliest form of realism is found in Edward Hallett Carr's The Twenty Years' Crisis. "In the field of action, realism tends to emphasise the irresistible strength of existing forces and the inevitable character of existing tendencies, and to insist that the highest wisdom lies in accepting, and adapting oneself to, these forces and these tendencies."
In other words, realism is a descriptive theory of how nations behave. There is an embedded logical argument that, given this pattern of behavior, the prescribed strategy is to behave similarly.
Realist theorists diverge on the causes of this pattern of behavior, and on the expected 'equilibrium' states of the world. For example, a defensive realist expects that arms races and mutually assured destruction keep the world locked in stalemate, with great powers expending military power to maintain this status quo (because they benefit most from it). In contrast, an offensive realist expects that great powers will actively undermine a peaceful status quo (because uncertainty trumps any degree of belief in a stalemate).
Reading Notes
The Tragedy of Great Power Politics, John J. Mearsheimer, 2014 (2nd ed.)