Let's forget what we learned about counter-insurgency again. Starbuck at Wings over Iraq puts out quite well that the US military tried to avoid doing anything COIN-like after the bad experience in Vietnam. They tried hard to forget the lessons and present politicians with reduced options to avoid another counter-insurgency campaign. Gen. Petraeus was so distinct (a real maverick as opposed to pols posing as such) precisely because he looked back and sought to learn the lessons of Vietnam and other such campaigns.
Why? Because he was smart enough to realize that the politicians would send troops into such situations, prepared or not. So, better to be prepared, right? One of the themes of this week in various blogs and tweets has been: insurgency has been the most frequent form of warfare over the past decades/century. Part of the recent insurgency "fad" (and I use "" sarcastically because this is one fad that is not going out of style soon) is precisely because the US and its allies have a commanding advantage in conventional warfare. The two Iraq wars of 1991 and 2003, the bombing campaigns of Kosovo and Libya, and even Operation Medusa in 2006 (where the Canadians faced entrenched Taliban who did not run--but died instead) serve to remind intelligent opponents that conventional confrontations are to be avoided.
Which ones that the US military (and others) have two choices: cede all military confrontations to the adversaries that chose not to stand and fight or learn how to do COIN as one of many techniques in one's repertoire. The former seems to be an unwise choice. Yes, COIN is hard, especially with dysfunctional allies, but the alternative really is to surrender anytime an opponent is smart enough to choose insurgency as their main approach.
I have speculated that one lesson from Afghanistan is that NATO countries will avoid alliance warfare due to problems revealed by the the caveats and other challenges (analyzed in an amazing book coming to a bookstore near year eventually) of multilateral military operations. But the reality is that politicians will want to use force when they face various pressures and they will often have a short memory about previous efforts.
So, the US Army can forget what it has learned in Afghanistan and Iraq, but they will just have to learn again and again. While COIN may be losing popularity, the lessons were learned the hard way--through the lives of Americans and Afghans and Iraqis (and our allies as well). It would be nice if we don't have to pay such a high price again to re-learn.