# Content The Correlates of War (COW) Project has utilized a classification of wars that is based upon the status of territorial entities, in particular focusing on those that are classified as members of the interstate system. Wars have been categorized by whether they primarily take place between/among states, between/among a state and a non-state actor, and within states. Within the COW war typology, an interstate, intrastate, or extrastate war must meet same definitional requirements of all wars in that the war must involve sustained combat, involving organized armed forces, resulting in a minimum of 1,000 battle-related combatant fatalities within a twelve month period. For a state to be considered a war participant, the minimum requirement is that it has to either commit 1,000 troops to the war or suffer 100 battle-related deaths. When Correlates of War scholars J. David Singer and Melvin Small first extended their study of war to include intrastate wars in Resort to Arms, they established the requisite condition that for a conflict to be a war, it must involve armed forces capable of “effective resistance” on both sides. They then developed two alternative criteria for defining effective resistance: “both sides had to be initially organized for violent conflict and prepared to resist the attacks of their antagonists, or the weaker side, although initially unprepared, is able to inflict upon the stronger opponents at least five percent of the number of fatalities it sustains.” The effective resistance criteria were specifically utilized to differentiate wars from massacres, one-sided state killings, or general riots by unorganized individuals. # Acknowledgements The dataset was created by Meredith Reid Sarkees, American University, and Professor Frank Wayman, University of Michigan-Dearborn, and published by the Correlates of War Project.