Productivity and the Declining Viability of Conquest
Robert Farley has a good post on the question of "Why is it that the United Kingdom, which is in an absolute sense far more wealthy now than it was in 1930, having difficulty maintaining a foreign deployment of about 10,000 total in Iraq and Afghanistan, while in 1930 it deployed many multiples of that total all over the world, plus colonial auxiliaries who were partially paid for by the Crown?" As he observes:
The relative increase in the effectiveness of insurgency strategies isn't just a consequence of the spread of the AK-47 or of the further development of nationalism in the non-western world; it's also a consequence of the fact that modern, wealthy states can now deploy far, far lower numbers of troops than they could fifty years ago. Indeed, in 1965 the United States (with a smaller and much poorer population in absolute terms) managed to deploy half a million troops to Vietnam while at the same time maintaining large contingents in West Germany and South Korea.
Farley gives some good answers to the question, but it's worth noting that this is part of a perfectly general situation. As technology improves, the average level of productivity goes up. And as productivity goes up, wages go up as well, at least over the long term. The wages go up, however, more-or-less across the board whereas productivity has only actually improved in the select areas that have seen meaningful improvement. As a result, things that are intrinsically labor-intensive tend to get more expensive and rarer over time, even as overall living standards go up.
A rich American in 2006 is way richer than a rich American in 1906, but the number of people employing large numbers of domestic servants is dramatically down. Similarly, it used to be that people of modest means by the standard of their time (to say nothing of our time) would own hand-crafted furniture that would be absurdly expensive in the modern day. Similarly, while the art of war is certainly enhanced by better technology, this falls overwhelmingly on the "blowing things up from a distance" side of the ledger. Controlling some conquered territory effectively still requires . . . lots of dudes walking around. But it's much more expensive to employ a bunch of dudes than it used to be, especially since the desire is to find sufficiently high-quality people that they can be trusted to operate the expensive and complicated equipment that's used for the "blowing thigns up" missions.