Is War an Instrument of Policy?

One of Carl Clausewitz's most famous dictums is that “war is an instrument of policy.” Yet, few writers over the past two centuries have attempted to define what is meant by the term “policy.”  Indeed, when doing so, most posit a definition that is so broad as to include virtually any type of motive for war: power politics, domestic politics, economics, irredentism, religion, etc. This is far too broad and tends to make the dictum meaningless. Moreover, many nations do not always wage war for such concrete reasons; rather, sometimes it is waged for cultural reasons such as pride, honor, revenge, fear, love, hate, or prestige.