What about the long peace that has held among the great powers for the better part of six decades? Would it survive in a post-American world? Most commentators who welcome this scenario imagine that American predominance would be replaced by some kind of multipolar harmony. But multipolar systems have historically been neither particularly stable nor particularly peaceful. Rough parity among powerful nations is a source of uncertainty that leads to miscalculation. Conflicts erupt as a result of fluctuations in the delicate power equation.You might also be interested in Kagan's other piece on Ameircan decline, in the New Republic.
War among the great powers was a common, if not constant, occurrence in the long periods of multipolarity from the 16th to the 18th centuries, culminating in the series of enormously destructive Europe-wide wars that followed the French Revolution and ended with Napoleon's defeat in 1815. The 19th century was notable for two stretches of great-power peace of roughly four decades each, punctuated by major conflicts. The Crimean War (1853-1856) was a mini-world war involving well over a million Russian, French, British and Turkish troops, as well as forces from nine other nations; it produced almost a half-million dead combatants and many more wounded. In the Franco-Prussian War (1870-1871), the two nations together fielded close to two million troops, of whom nearly a half-million were killed or wounded.
(Image: "Multipolar systems have historically been neither particularly stable nor particularly peaceful. Nearly a halfmillion combatants died in the Crimean War," depicted in "The Taking of Malakoff" by Horace Vernet, pictured here. Getty Images/The Bridgeman Art Library)