Committing extra forces is the best that can be done in an excruciating situation. At the moment the US and its allies are losing. It is that simple. Mr Obama's options are essentially to pull out altogether, conceding defeat in his necessary war; maintain roughly the existing commitment, but define success way down, narrowing the mission to arm's length harassment of the enemy; or provide the resources his military commanders say are needed to make a success of counter-insurgency, while building up Afghanistan's own security forces. It seems he will do the last, or close enough to it to avoid accusations of splitting the difference.
The lines between these options are blurry, to be sure. One could argue endlessly - indeed, the administration has been arguing endlessly - about what counter-insurgency means. Even this trebling of US forces will be too small a commitment to smother the Taliban. The number of troops is less important than what they will be asked to do. The administration will still have to ensure that its augmented forces are not over-stretched, as they have been for the past eight years.
That will mean ceding control of some areas to the enemy, and coming to terms - that is, bribing - some tribal leaders who could go either way. The best outcome under the counter-insurgency route will still be very messy.
Then why try? For the two reasons the administration advanced in the spring, when it announced the previous, inadequate, escalation. A failure to secure Afghanistan creates a zone in which al-Qaeda can recover and flourish. And defeat in Afghanistan will most likely strengthen the Taliban in Pakistan, posing an even worse threat to the west. Here is a third reason. The US and its allies owe the main victims of this calamity - ordinary Afghans - our best efforts to rebuild their country. We are calling on them to take sides. We cannot succeed unless they do so, at great personal risk. That creates an obligation.