Wealth constitutes both the apprehension and the incentive to invasion.

Where two or more states have equal means of war, with incentives very unequal, an equilibrium cannot exist; for danger and temptation are no longer opposed to each other.

The preparation of states may, therefore, be equal without being equivalent, and the smaller of the two may be most liable to be drawn into a war without the means of sustaining it.
The numerical relation between the entire population of a state, and the armed forces which it can maintain, must evidently vary with the wealth and pursuits of the people.

Adam Smith thinks that a country purely agricultural may, at certain seasons, furnish for war one-fifth, or even in case of necessity one-fourth, of its entire population.

A commercial or manufacturing country would be unable to furnish any thing like so numerous a military force.