A world war is a conflict involving many or most of the world's most powerful and/or populous countries, spanning multiple continents and theaters, though no one can agree as to what truly constitutes a world war. Many wars have been claimed by historians to be on a global scale, including ones which involved most or all of the known world rather than actually taking place on the world scale.
Proposed World Wars
Most of the conflicts suggested as being world wars have been between the European and Middle Eastern powers, from Alexander's conquests to the Byzantine-Sassanid wars to the Crusades to the Ottoman-Habsburg wars. Among the more frequently claimed, though, is the Seven Years' War, which took place on five continents in the later 18th century. The Coalition Wars are sometimes considered one long conflict taking place over a similar geographic span, though mostly occurred in Europe.