“ The art of war, however, as it is certainly the noblest of all arts, so, in the progress of improvement, it necessarily becomes one of the most complicated among them. ”
 Adam Smith, The Wealth of Nations (1776). copy citation

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“In the republics of ancient Greece and Rome, during the whole period of their existence, and under the feudal governments, for a considerable time after their first establishment, the trade of a soldier was not a separate, distinct trade, which constituted the sole or principal occupation of a particular class of citizens; every subject of the state, whatever might be the ordinary trade or occupation by which he gained his livelihood, considered himself, upon all ordinary occasions, as fit likewise to exercise the trade of a soldier, and, upon many extraordinary occasions, as bound to exercise it.
The art of war, however, as it is certainly the noblest of all arts, so, in the progress of improvement, it necessarily becomes one of the most complicated among them. The state of the mechanical, as well as some other arts, with which it is necessarily connected, determines the degree of perfection to which it is capable of being carried at any particular time. But in order to carry it to this degree of perfection, it is necessary that it should become the sole or principal occupation of a particular class of citizens; and the division of labour is as necessary for the improvement of this, as of every other art.” source