Keep in mind that until fairly recently, the conventional wisdom was that we had already won in Afghanistan and could never win in Iraq. Now we hear the reverse, but the new zeitgeist is no sounder than the old. We can win in Afghanistan, as we are now winning in Iraq.

The key is for policymakers to ignore the naysayers. They will get louder over time, because, just as in Iraq, a surge in the number of U.S. soldiers in Afghanistan will inevitably bring about a short-term spike in casualties. But if President Obama doesn’t lose his nerve, the odds are that a classic counterinsurgency strategy, supported by adequate troop levels, can turn around another failing war effort.