This week’s confrontation between a senior Army general and the president of the United States may have signaled the beginning of the end of the war in Afghanistan. In a year or two, President Obama will be able to say that he gave the conflict his best shot, reshaping the strategy and even putting in charge his top guy, the general who led the surge in Iraq — but that things still didn’t work out.

Then he can begin pulling out.

This is not a vote of no-confidence in Gen. David H. Petraeus, whom the president has selected to lead the U.S. effort in Afghanistan, replacing the disgraced Gen. Stanley A McChrystal. It is a simple recognition that the conditions Petraeus enjoyed in Iraq are far from present in Afghanistan, and that the key skills he brought to bear in the first war won’t help him as much in the second.