The fear that a conflict will spread is often used as a justification for "peacekeeping" operations. But why and under what conditions is war likely to widen? When are concerns warranted and justified? This book answers these important policy questions. The book offers a theoretical explanation for war widening based on the decisiveness of warfare in a given era. It argues that conflicts are most likely to spread when the effects of warfare are limited, as states seek limited gains with a low cost. In an era where warfare is decisive, in other words, an era of total war, wars are less likely to widen. By understanding whether a war occurs in an era of total war or limited war, we can then assess how likely that conflict is to spread. The explanation of war widening is developed through four historical cases: The Seven Years War, the French Revolution/Napoleonic Wars, the Crimean War and World War I. These cases capture both eras of limited war, total war and the transition between them. Finally the author looks to the future, to foresee what developments might limit the costs of warfare and make future conflicts likely to spread.