America took a specific approach in the aftermath of 9/11. Neoconservative American leaders believed that terrorism was fundamentally a type of proxy warfare carried out by hostile states. So we responded by turning two anti-American regimes into failed states between 2001 and 2003. (Well, one was already pretty much failed, but we double-failed it.) That was the wrong approach. An alternative approach, advanced by some at the time, was to view terrorism as a phenomenon that mostly emerges out of states that are already failed or failing. The relevant military approach is counterinsurgency, to help turn failed states into sustainable states, rather than the other way around.
That approach has been much more successful than the Rumsfeld strategy. It may be too late, at this point, to salvage anything like the goals which a counterinsurgency approach would have targeted in Afghanistan in 2002. And maybe we wouldn't have achieved those goals anyway. But it would have been the right way to fight the war, and we should keep that in mind the next time we get into a similar conflict. John Nagl's book on counterinsurgency analogises it to "learning to eat soup with a knife." Matthew Yglesias points out that eating soup with a knife is incredibly tedious, and should probably be avoided where possible. Even so, in case we ever do need to eat soup with knives again, it'd be a good idea to remember the tricks we've learned so far, and that banging away as hard as you can doesn't work.