The Importance of Tribute

Gaining tribute and booty is definitely an incentive of conquest, but it is also a requirement for sustained expansion. As the state grows through military acquisitions, the need for resources also increases, especially for materials used in warfare, food for the soldiers and other resources for governing the new regions of the state. This demand is shown by the way the Assyrians dealt differently with different tribes and cities they conquered. By dealing more forcefully with some and more leniently with others, they maintained a steady flow of tribute and troops to sustain their expansion.

What's interesting is that in expanding in search of greater bounty or troops, their conquests also expended both these resources. Therefore, for a state to sustain a territory expansion, the state would have had to have gained enough surplus resources to pay for the expenses of the campaign, or continue to expand.