The Art of War by Sun Tzu

II Waging War

11. On the other hand, the proximity of an army causes prices to go up; and high prices cause the people's substance to be drained away.

12. When their substance is drained away, the peasantry will be afflicted by heavy exactions.

Notes by Zephyrist:

The first one is quite simple, armies eat resources causing prices to rise generally ruining the country and impoverishing the people. In extreme this leads to starvation and desolation of the land.

Apparently, I had to look this up, exaction means a heavy an unjust demand. Basically the local populace aren't really obliged to support the army but end up doing so. As an extension to this the local populace will probably resent the army which as Sun Tzu has mentioned previously will lessen the chances of success.

To summarise Sun Tzu believes that large armies are to be used, not just hang around waiting for a battle.