At the end of my long-winded, possibly mind-numbing expository piece on the US empire, I noted that I'm beginning to suspect that the US has gotten in to the global empire game too late.
Before I get in to that, I'd like to address a point that Patrick Porter at Oxblog notes, referring to Niall Ferguson: Historically, the fall of empires has been disastrous. We don't even need to look at Rome - rather, the interwar period (1919-39) can be characterized as a period between the decline of old empires and the rise of new hegemonic powers. World War II, in this portrayal, merely acted as a catalyst to finish off the British, French, and other European colonial empires, and inaugurated the American and Soviet Empires.
I'll certainly acknowledge that the decline of American hegemony - if it's happening at all - carries real risks. That said, I don't think we're likely to see the kind of spasms of carnage that World War II represented. (This is quite different from my belief that major-power war is a probability in the next century.)
World Wars I and II were unique in that they were the first - and thus far, only - time in history that modern, industrialized nation states engaged each other in total war. What really sets the war against Hitler apart from the war against the Kaiser is that while technology had changed incrementally, tactics had changed dramatically - while the Great War had chewed up men for three and a half years before the front moved in earnest, Blitzkrieg showed that the front could move rapidly and without warning.
More importantly for empires, rapid industrial war showed that these wars could be brought to a decisive conclusion. Before they were turned back, the Nazi armies got to within a line-of-sight of the Kremlin towers in Moscow, having surrounded St. Petersburg and nearly encircled Stalingrad. (Of course, "nearly" proved to be disastrous.) While Operation Barbarossa ultimately failed, it's worth pointing out that, in the hands of competent leaders, the Wehrmacht may very well have crushed the Soviet Union. I find it difficult to believe that, had that been the case, any western victory against Germany would have been possible.
But is a similar story conceivable today? I would say no, for important technological reasons. The first - and most obvious - is the proliferation of nuclear weapons. Consider for a moment what the likely result of a second German offensive against Russia would be today. If the Russian leadership were placed in as precarious a position as in 1942-43, I think we can safely assume Berlin would be reduced to radioactive rubble, whether or not Germany had its own deterrent. The same is true of any nation that posesses a deterrent capability.
If we look at the states that have nuclear weapons today, it includes all the major powers, and most of the candidates for future great-power status (China, India, Russia, the US, and of course the EU has two nuclear powers.) Certain countries in South America and Africa either had the technology for nuclear weapons and gave it up (South Africa and Brazil come to mind.) So any future war between the major powers is unlikely to threaten their survival, because any power that threatens the survival of their rivals is putting its own life at risk. This was perceived early on in the nuclear age, by no less than George Orwell in 1984.
There are two other things that have proliferated in the world that have greatly reduced the vulnerability of non-nuclear states to imperial conquest. The first comes from the 19th century, and the second is very much a child of the 20th.
The old European empires really had it easy - they had technological superiority over their conquests of a quality that the US - even today - could only dream of. The colonization of Africa, the dismemberment of China, even the imposition of relations with Japan, all occurred in an environment where the then-modern weaponry of machine guns and railroads made resistance to European advances using traditional, frankly primitive, means all but impossible. The western countries then set up their own states along western models, and proceeded to pillage for almost a century.
This was sold as charity work of a sort, but in one sense it was also military aid: One of the west's best advantages was forms of social organization that made industrialization and modern war possible. And we didn't just provide those forms to the third world - we forced it on them.
Even where we didn't explicitly force modernism on the colonies, they learned quickly enough that it was necessary if they were ever going to be free. So in China, the independence movement adopted American Republicanism as its referred form. Disillusioned with the west after Japan's imperialism was accepted after World War I, Chinese nationalists later turned to Soviet models - even the Nationalists effectively became a Bolshevik party in form, if not in their aims.
The second proliferation - this one of the 20th century - was of western military technology. When Britain went in to Africa, it was fighting Zulus with spears (initially.) When the western powers went to Asia, they found more advanced nations, but still inflicited humiliating defeats on the "natives". The situation - globally - has now been reversed. Wherever you go in the world, you can find modern automatic rifles, explosives, and assorted other small weapons.
A lot used to be made of America's technological advantage over its rivals - the Hyperpower concept. Certainly, America's technological superiority over the Iraqi army was impressive, but the gap between an M-16 and an AK-47 simply isn't large enough to give America the kind of advantage that the colonizers used to have.
This has meant problems for colonial emires in the post-WWII era. The important thing to remember about Vietnam, to me, is not that the US was beaten with a guerilla war. The crucial thing is that the Vietnamese beat the French in a conventional battle at Dien Bien Phu in 1954. They did so with major casualties, of course, but they beat the French with modern armies and conventional weapons.
We don't yet see this kind of scenarion in Iraq, largely because the there's still a major debate in the muslim world over whether or not to adopt western social forms - indeed, the debate seems to be heading decisively against. Nevertheless, religiously-tinged nationalism in the muslim world is nothing new (see Iran.) In Iraq, you can see a divide between the Sunni resistance, which seems intent on carnage, while the Shia have - more or less - held back.
What can we expect if the Shia clerics decide to move against the US in Iraq? Say, in a response to an American attack on Iran? Well, Steve Gilliard is predicting Chosin Reservoir II - a sequel to the humiliating American retreat (retreat, hell!) just ahead of the Chinese army in Korea. But in that case, the army had somewhere safe to retreat to - where does the US go in Iraq today? South to Kuwait? Saudi Arabia? North to Turkey (maybe)? That's possible, but it's just as likely that we'll see US forces pinned down in their bases, waiting for airlift out - Dien Bien Phu, but this time the Viet Minh (or Mahdi Army) will have RPGs to knock helicopters out of the sky.
All this is highly speculative, but it illustrates the problems of overseas adventurism nicely. What can a modern military - even when it's competently led - do against a nation in arms with assault rifles? Well-armed, modern soldiers are incredibly expensive, which limits their numbers. And they're incredibly effective - capable of inflicting greater casualties by 10:1 or more. But what if they're outnumbered by 20:1? 30:1? There's 150,000 soldiers in Iraq, but maybe half of those are combat-trained. How many Iraqi men know how to hold a gun? What about the women? There's no shortage of weaponry for them. I'd be surprised if there aren't at least 100 armed and angry Iraqis for every US combat soldier - say, 7.5 million?
This isn't just a problem for the Americans, though they're suffering at the moment. It's a problem for the Russians, who are increasingly unable to win in Chechnya. It will be a problem for the Chinese if the Uihurs or Tibetans ever move against them (though I'm not optimistic for the Uighurs or the Tibetans in that case.) In sum, no modern army can hope to achieve the kind of dominance that conquest requires, at least without inflicting the kind of casuaties that western democracies blanch at.