Even more than healthcare, the war in Afghanistan will decide whether Barack Obama succeeds or fails. On Tuesday night, in a speech at the West Point military academy broadcast across the country, Mr Obama will lay out his plans.

Mr Obama already owns this “necessary” war, as he has called it, contrasting this battle with his predecessor’s supposedly needless war in Iraq. But Tuesday’s speech will expunge the last particle of doubt on that score. He is expected to commit upwards of 30,000 more US troops to the mission.

Added to the 21,000 extra soldiers he has already assigned, plus support forces, this increase would roughly treble the Bush administration’s commitment. Last week, again to mark the contrast with the previous administration, Mr Obama promised to “finish the job”. Muscular talk, and making the announcement at West Point underscores it. If health reform goes wrong, there will be others to blame. If this war goes wrong, it will be all his fault. It is Mr Obama’s biggest bet by far.

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.

None of these arguments is decisive. A point may come when the US is doing more harm than good, or when the Afghans themselves want us out. The case for gradual withdrawal, starting now, is not obviously wrong. This is not a necessary war. It is a war of choice, and a finely balanced choice at that. This makes Mr Obama’s political difficulty acute.

Parallels between Afghanistan and Vietnam are impossible to ignore. The most pressing is that the US loses wars like this at home. A bigger effort in Afghanistan can be sustained only as long as the country supports it. The slimmest plurality of voters favours sending extra troops. As with Vietnam, most Americans are unsure why their sons and daughters are dying in Afghanistan. The administration’s unduly protracted debate over what to do has sent the message that it too is unsure. Shallow support for the war suggests that one spectacular Taliban strike might flip the balance of opinion – and, with or without extra forces, the US would then be back on the path to defeat.

It gets worse. Mr Obama’s own party opposes the policy he seems to have chosen. Last week leading Democrats called for a war tax to cover the cost of the country’s expanding commitments. Not exactly helpful: but they are right that operations in Afghanistan are enormously costly, in financial as well as human terms. The administration says it costs $1m a year for every extra soldier. An additional 35,000 troops would cost $35bn a year – enough to buy a lot of health reform.

For his narrow margin of support on extra forces Mr Obama relies on Republicans, with whom he has fallen out bitterly on every aspect of domestic policy. The president’s approval rating continues to slide. The mid-term elections are in sight, and Democrats are anxious. They have reason to be.

In short, the test for Mr Obama could hardly be more demanding. Having made his decision, he must get the country behind it, without making promises he cannot keep or sending messages that encourage the enemy. Pledging to “finish the job” within a fixed period and then pull out – the obvious way to sugar the pill of escalation – would do both.

Since taking office, Mr Obama has been a less effective leader than many of his admirers, myself included, had hoped. On many issues, he has simply chosen not to try. On Afghanistan, standing aside is not an option. We will see what kind of president he is.

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