Well, firstly, from what we know of the internal debates surrounding the entry of classical states into war from textual sources, plunder doesn't really get thrown around a lot. Livy doesn't have the
Patres Conscripti saying, "We need to go to war against the Carthaginians in order to gain spoils that will keep our military-agricultural complex going" before any one of the Punic Wars, for instance. There's just not a whole lot of support in
favor of that sort of an interpretation.
And classical rulers definitely went to war for territories that were either poor or virtually depopulated, for a variety of reasons (geostrategic gain has been attested, for instance, in the case of Philippos V's invasion of inland Illyria, and then there's prestige, of course) - moves that would be nonsensical if their militaries were so overlarge that they could not be supported by the states that ran them, as the Schumpeterian model goes.
This, of course, does not mean that individual campaigns might be conducted to make a profit, because we do have clear instances in which this might plausibly be the case (although it's not directly attested; again with Phil, we see his eastern Aegean campaigns generating some revenue, which ended up going to finance a fleet buildup to add to the navy he'd already seized from the Ptolies at Chios). And of course no ruler would ever turn down loot of any kind if he were able to get it in the course of a campaign. But as far as motivation...I guess what's most striking is what's
not there about looting.
