Not the only example of this: Towards the end of the Vietnam war, the US had developed a fairly comprehensive counter-insurgency (COIN) program. By Iraq 2, they

Not the only example of this: Towards the end of the Vietnam war, the US had developed a fairly comprehensive counter-insurgency (COIN) program. By Iraq 2, they were pretty much only equipped for state on state warfare again and had to relearn COIN - and were actually doing an okay job by 2008. Ironically, given the withdrawal from Iraq, they're probably going to have to relearn it again the next time they're forced to put boots on the ground. It happens quite regularly. Military forces have to adapt to the problem of the day.