The resources for war are acquired by conscription, taxation, confiscation, and inflation. Without cannon and cannon fodder, there are no wars. With their aura of legitimacy, democracies are particularly adept at utilizing all these means. Since citizens tend to identify with the democratic state, there is usually little trouble conscripting troops and confiscating the economic resources required for war. True, democracies rarely fight each other.
Democracies do, however, like to fight non-democracies.
Suppose they gave a war and the democracies didn t show up. But, you may say, the non-democracies started all these wars. The United States was looking for any excuse to get into World War Part I against Germany. Democracies, however, do not seem to mind their own business. The United States has always seemed to get attacked just when its leaders were plotting to drag the nation into war by any means possible Fort Sumter, Remember the Maine, the Lusitania, etc.