A Century of Coalitions in Battle

Under what conditions do battlefield coalitions fight as greater or less than the sum of their parts? Introducing the Belligerents in Battle dataset, which contains information on actors fighting in 492 battles during interstate wars waged between 1900 and 2003, this chapter presents, for the first time, a portrait of the universe of battlefield coalitions. Battlefield coalitions win more often and suffer fewer casualties than belligerents fighting alone. Battlefield coalitions including forces fielded by the United States, states with pre-existing treaty agreements, and democracies are particularly powerful. By contrast, battlefield coalitions that include non-state actors lose the majority of their fights.