The Wealth of Nations by Part 5 Chapter 1 Page 21

agriculture and manufactures, and which in reality is no more than the accumulated produce of those improvements, provokes the invasion of all their neighbours. An industrious, and upon that account a wealthy nation, is of all nations the most likely to be attacked; and unless the state takes some new measures for the public defence, the natural habits of the people render them altogether incapable of defending themselves.

In these circumstances there seem to be but two methods by which the state can make any tolerable provision for the public defence.

It may either, first, by means of a very rigorous police, and in spite of the whole bent of the interest, genius, and inclinations of the people, enforce the practice of military exercises, and oblige either all the citizens of the military