In international relations, defensive realism is a variant of realism. Like realism, defensive realism looks at states as rational 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 where one state's drive to increase its security can, because security is viewed as a zero-sum game, result in greater instability as its opponent(s) respond to their resulting reductions in security.

There are several versions of defensive realism, including Offense-Defensive theory which states that there is an inherent balance in technology or doctrine that favors either the attacker or defender in battle. To be theoretically useful this balance must both exist and be measurable. Offense-Defense theory explains the First World War as a situation in which all sides believed the balance favored the offense but were mistaken.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Defensive realism".