Throughout history, the character and organization of armies have changed. Social and political aspects of nations at different periods resulted in revision in the makeup of armies. New weapons influenced the nature of warfare and the organization of armies. At various times armies have been built around infantry soldiers or mounted warriors or men in machines. They have been made up of professionals or amateurs, of mercenaries fighting for pay or for plunder, or of patriots fighting for a cause. Consideration of the development of armies must be made in the light of the times in which the particular army was forged and the campaigns that it fought.

Armies and warfare are treated in a number of articles in the Macropaedia. For general considerations of armies and warfare, see War, Theory and Conduct of (war); War, Technology of (military technology). For the history of major modern conflicts, see World Wars (World War I); international relations; European History and Culture (Europe, history of). See also the Index for individual wars, battles, and other military subjects and for individual countries and their histories.