In international relations, defensive realism is a variant of political realism. Defensive realism looks at states as socialized players who are the primary actors in world affairs. Defensive realism predicts that anarchy on the world stage causes states to become obsessed with security. This results in security dilemmas wherein one state's drive to increase its security can, because security is zero sum, result in greater instability as that state's opponent(s) respond to their resulting reductions in security.
Among defensive realism's most prominent theories is that of offense-defensive theory which states that there is an inherent balance in technology, geography, and doctrine that favors either the attacker or defender in battle. Offense-Defense theory tries to explain the First World War as a situation in which all sides believed the balance favored the offense but were mistaken.
Defensive structural realists break with the other main branch of structural realism, offensive realism, over whether or not states must always be maximizing relative power ahead of all other objectives. While the offensive realist believes this to be the case, some defensive realists believe that the offense-defense balance can favor the defender, creating the possibility that a state may achieve security. A second-strike capable nuclear arsenal is often understood to indicate the supremacy of the defense in the offense-defense balance, essentially guaranteeing security for the state which possesses it. Yet in a multi-polar world a second strike capability does not provide the same guarantees that it did during the bi-polar Cold War period. Some defensive realists also differ from their offensive counterparts in their belief that states may signal their intentions to one another. If a state can communicate that its intentions are benign to another state, than the security dilemma may be overcome. Finally, many defensive realists believe that domestic politics can influence a state's foreign policy; offensive realists tend to treat states as black boxes.
In modern times, several economic and political groups are known to benefit from the effects Defensive Realism, in terms of both the economic activity generated in delivering the resources or technology needed to increase a particular state's own security, as well as the positive feedback effect caused by the perceived destabilization to an opponents own security by comparative observation.
A more recent attempt to state defensive realism systematically and rigorously is Shiping Tang, A Theory of Security Strategy for Our Time: Defensive Realism (Palgrave, 2010), although Tang has denied that he is a defensive realist per se.
See also 
- See Charles Glaser and Chaim Kaufmann, "What is the Offense-Defense Balance? International Security 22 (Spring 1998), 44-82
- Charles Glaser is a key proponent of this idea. See Charlie Glaser, "Realists as Optimists: Cooperation as Self-Help," International Security 19 (Winter 1994-95), 50-90.
- Stephen Walt, "International Relations: One World, Many Theories," Foreign Policy 110 (Spring 1998), 29-45.