Military force played three important roles in early civilizations. It secured and defended the frontiers of states; it enriched powerful rulers by enabling them to exploit peripheral areas that contained valuable natural resources, control lucrative trade routes, and impose tribute on conquered peoples; and it maintained internal order – suppressing rebellions, ensuring the collection of taxes, maintaining social peace, and protecting the power and privileges of the upper classes. No clear distinction was drawn between military and police forces. Because order and prosperity were important for everyone, military strength was endowed with great political and religious significance.

Armed conflict was invariably destructive of human life and property, but wherever it occurred in early civilizations it was preferable to belong to a strong and disciplined polity than to a weak one. Military success offered opportunities for personal enrichment and prestige to kings and commoners alike. Warfare was surrounded by elaborate rules of conduct which sought to limit unforeseen consequences, but these rules were often disregarded in extreme situations or when fighting enemies who did not understand them. The Aztec and Inka both requested the target of a planned attack to submit peacefully. Aztec ambassadors invited rulers to become friends of the Aztec king, allow trade, and pay nominal tribute. If this offer was not accepted, a short time later their Texcocan allies would threaten the targeted state with severe punishment if it did not submit.