The Wealth of Nations by Part 5 Chapter 1 Page 19

exercising a great number. But it is the wisdom of the state only which can render the trade of a soldier a particular trade separate and distinct from all others. A private citizen who, in time of profound peace, and without any particular encouragement from the public, should spend the greater part of his time in military exercises, might, no doubt, both improve himself very much in them, and amuse himself very well; but he certainly would not promote his own interest.

It is the wisdom of the state only which can render it for his interest to give up the greater part of his time to this peculiar occupation: and states have not always had this wisdom, even when their circumstances had become such that the preservation of their existence required that they should have it.