Lack of intelligence, lack of division of labor and violent ideologies are three factors which contribute to states, wars, and imperialism. Fighters in wars were vassals of the Lords, or mercenary groups who could be hired. Fights were frequent but small and they had rules of knightly honor.
Ideological wars are the most brutal. Religious uprisings were used by princes as ways to grab properties and to form alliances. By 1648, following the Thirty Years War, the German territories had lost about one-third of their land. States grew. States as institutions have natural aggressiveness. States competed against other states in order to keep or gain taxpayers. Standing armies were created. Monarchical warfare followed. Innocents and citizens were not targeted. Many victories were attained without loss of life.
The French Revolution opened up conscription. Democracy said you had to serve the state. It became difficult to stop wars. Wars became total wars. The United States has always been engaged in imperialistic aggression.
Lecture 9 of 10 from Hans-Hermann Hoppe's Economy, Society, and History.