it might be impossible for us to actually follow wise policies in a sustained way or are we destined to flit from error to error until our national power is so badly compromised that we have few options left? As someone who likes to think of himself as involved, in a small way, in trying to get the country on a better course I don’t think I have any option other than to say the answer is “no.”Hah. And here I am, thinking the evidence of the last near-decade is incontrovertible proof that the answer is in fact "yes". Leaving aside my pessimism that America is actually capable of long-term self-governance, I'm intrigued by Yglesias' agreement to Justin Logan's assertion that structural elements of the international arena aren't already working to limit America's power in substantial ways.
The existence of nuclear weapons, held in the hands of all major powers (and some minor ones too) means that Big War is either a) behind us or b) likely to end faster than it would take to mobilize the tanks. In either case, we're not going to see armored columns slicing through Poland again any time soon. Nor will we see conventional warfare between China and the US last longer than it takes for the first ICBM to leave an IR signature on satellite imagery. This is also the standard realist explanation for the long peace of the Cold War: nuclear weapons binding the hands of the superpowers.
On the other end of the spectrum, widely dispersed and proliferated small arms are now of a quality that can, in the hands of amateurs, bleed and preoccupy an invading army for years, preventing even the most basic of objectives from being accomplished until such time as the invader gives up on all of the nominal political goals of the original invasion. See Iraq, 2003-present. I honestly have lapsed in my IR readings over the last few years, but this seems to me at least as important a change in the international arena as the advent of nuclear weapons: Iraqi insurgents, with no outside sponsor of any international consequence, have totally negated any and all US efforts in Iraq. It's a rather striking thing that after fighting the Sunni of Anbar province for years, America was far more effective in buying them off. This says quite a bit about American military effectiveness, and about the effectiveness of modestly-armed and -organized forces. None of it is good for notions of American primacy.
As I said, I haven't read much in the way of IR journals for the last few years, but if realists are going to use nuclear weapons to explain one aspect of international behaviour, it seems reasonable to use small arms in a similar fashion. (I'm sure this isn't a novel insight.) America can, in a very narrow set of circumstances, be very effective. But make no mistake: the range of situations where America can intervene effectively is extremely narrow. Basically, using American power as it exists against any country larger than Serbia risks either nuclear retaliation or a prolonged, awful war of occupation where insufficient American numbers are eaten alive by motivated and passably-armed insurgents.
Now, American leaders clearly don't perceive the threat from small arms as clearly as they perceive the threat of nuclear weapons, but that doesn't make it any less. (Mao was ambivalent about the threat of nuclear weapons, after all. That didn't make him right.) So congratulations, America: the most powerful country on Earth has enough military power to reliably smack around the Serbias, Albanias, and East Timors of the world. (And can mount expeditionary forces smaller than those of 1930s Italy, btw.) But even a sanctions-hobbled, dysfunctional trinational state like Iraq poses a problem that US power can't really solve.
For a more concrete example, take a look at the options with Iran. Answer: there aren't really any. Either Iran gets the bomb, or it doesn't, but the policy options open to America are basically 1) ineffectively bomb a bunch of stuff, and Iran gets the bomb a few years later than otherwise, or 2) ask really nicely in a variety of ways for Iran to please stop. I'm an ardent supporter of policy option #2, but it's not exactly the roar of the lion, y'know?
Then there's the hypothetical: America could, in some fantasy situation, afford to raise a much larger army and (perhaps) successfully occupy a country like Iraq. Uh, maybe. But here again the existence of nuclear weapons means that nobody has any interest in the care and feeding of large standing armies, because you'll never get to use them. So it's unlikely that the American people will support a force large enough to back up the Primacy strategy unless they feel directly threatened by another conventional power -- in the neocon dreams, it's always China. But by the time that ever becomes a reality, yes, we will have passed out of unipolarity and in to bipolarity, or multipolarity.
So: the primacy moment was never really here, or if it ever was it disappeared sometime soon after the boys came home from Iraq in 1991.