science, when the event of war ceases to be determined, as in the first ages of society, by a single irregular skirmish or battle, but when the contest is generally spun out through several different campaigns, each of which lasts during the greater part of the year, it becomes universally necessary that the public should maintain those who serve the public in war, at least while they are employed in that service. Whatever in time of peace might be the ordinary occupation of those who go to war, so very tedious and expensive a service would otherwise be far too heavy a burden upon them. After the second Persian war, accordingly, the armies of Athens seem to have been generally composed of mercenary troops, consisting, indeed, partly of citizens, but partly too of foreigners, and all of them equally hired and paid at the expense of the state.