Offense-defense theory argues that the relative ease of offense and defense varies in international politics. When the offense has the advantage, military conquest becomes easier and war is more likely; the opposite is true when the defense has the advantage. The balance between offense and defense depends on geography, technology, and other factors. This theory, and the body of related theories, has generated much debate and research over the past twenty-five years.This book presents a comprehensive overview of offense-defense theory. It includes contending views on the theory and some of the most recent attempts to refine and test it.