Reading through Roman history right now, and there's huge amounts of governors, magistrates, and senators that all seem to jump at the chance to invade anyone, simply for the profits that they'll gain from 'conquest' and 'plunder'. Patricians are frequently claimed as taking out huge loans and mortgaging their property to campaign for an election as consul, with the loans being paid back relatively quickly due to the money gained from conquest.
Skip ahead to today, and it sure doesn't seem like you can get much money doing something like invading Afghanistan, which has practically no resources or technology of any kind to be interested in.
So I'm wondering if I'm actually completely wrong and war is actually still profitable under certain circumstances (Iraq-Kuwait comes to mind), or there really was a turning point in militarization sometime between the Roman Republic and today when nations were simply too strong to be invaded at a profit.
The activity of war itself has long been considered unprofitable, especially when prolonged. Sun tzu says, bluntly, "there is no instance of a country having benefited from prolonged warfare" and stresses to his readers that entering a war without the possibility of quick success is ruinous. The exploitation of conquered territories is where you have a chance to make some cash.
Additionally, consider that individual soldiers may profit in looting or plundering and clamor for the chance to do so even when the state would not get much benefit.
Finally, Roman leaders were particularly incentivized to acquire new territory both because it carried enormous prestige to do so and because of the nature of their governance. When you were appointed to administer taxes for a territory you paid the state a fee and were otherwise free to wring as much cash out of it for yourself as you liked. This was incredibly profitable.