Often opposing leaders or governing bodies get other people to fight for them, even if those fighting have no vested interest in the issues fought over. In time it became practical for some people to have warfare as their sole occupation, either as a member of a military force or mercenary. The original cause of war is not always known. Wars may be prosecuted simultaneously in one or more different theatres. Within each theatre, there may be one or more consecutive military campaigns. Individual actions of war within a specific campaign are traditionally called battles, although this terminology is not always applied to contentions in modernity involving aircraft, missiles or bombs alone in the absence of ground troops or naval forces.