Wars of conquest obsolete?

I like this David Weigel post over on Sullivan which quotes these two Steve Sailer posts. Sailer talks about the obsolesence of old-style warfare, where you send your army over to conquer territory:

Back in the good old days of James K. Polk, starting a cross-border war for lebensraum, minerals, and strategic harbors, such as the Mexican-American War, could be highly profitable, especially, as in the case of Mexican California, when the target was ridiculously underpopulated.

In the 21st Century, however, there just aren't that many such worthwhile targets laying around. Siberia, perhaps, but not too much else.

I think our invasion and occupation of Iraq is an illustration of this principle in action. It's just not worth it to invade and conquer someone anymore. Even Iraq which possesses vast amount of the world's most precious resource (oil) has been a huge economic negative for our country:

Minerals, other than oil, just aren't that important economically anymore. And we're spending 50% more on occupying Iraq each year than all the whole country's current oil production is worth at $70 per barrel even if we stole every drop.