What makes for innovations in war-making? The answers of this careful study, based primarily on American military experiences this century, run refreshingly against intuition: innovation seems easier in peacetime than war, for the fog of the latter covers all; it is no harder during periods of budgetary austerity than in flusher times; it is neither much connected to better intelligence about would-be foes nor much influenced by civilian leaders or thinkers. Those answers bear heeding now that the United States can no longer afford to build everything and then see what works.