I've been reading a lot of Charles Tilly lately. One of my main areas of interest is how the state essentially monopolized non-state violence (mercernaries and pirates, who used to do all the real fighting) and how that it is reversing.
For instance, one of the reasons for the rise of mercenary armies in Europe was that it was too expensive to maintain a standing army, it was much easier to buy one, pre-made, and roll with it. Especially due to the rules which bound feudal society, which essentially left knights with 84 days of the year they could actually fight on (so long as they didn't use: crossbows, great big spiked balls, poisoned weapons or a whole host of other techniques which also constrained them). Equally, asking a knight to go to war on behalf of his lord meant he would make an economic loss, because he'd call up the serfs etc who worked his land. Even looting etc wouldn't really make up for this loss.
The downside was, of course, it gave people like Albrecht von Wallenstein, or William Kidd, more power and men under arms than the kingdoms of Europe, or the colonies of North America.
Not surprisingly, the rise of modern mercenaries, private military companies, has also coincided with an increase in the cost of warfare. Multiple studies have shown that, despite their incredible contract fees, companies like Blackwater are still distinctly cheaper to field than US armed forces.
This also overlaps with the theory of Hedley Bull, to a degree, who argues that globalization has in effect created a new medievalism, with its overlapping non-state authorities. The cause and effect, as well as associated processes, are still very much up for debate.