In sending in a lightning strike force of 4,000 Marines to clear the Taleban from Helmand, Barack Obama is taking the riskiest strategic decision yet of his presidency. Operation Khanjar — “Strike of the Sword” — had been long prepared, with the announcement of 17,000 extra troops for Afghanistan, the steady build-up of 8,500 Marines in the province in the past two months and the tight co-ordination with the Afghan and Pakistani armies. But as thousands of US troops stormed into the Helmand river valley yesterday, they had all the elements of surprise, speed and overwhelming force, the classic elements of military success. If the strategy works, the operation could mark a turning point in the war. If it fails to stem the reverses and the bloodshed, it could demoralise Nato further, accelerate Afghanistan’s downward spiral and seriously damage Mr Obama’s authority at home and overseas.

The goal of the operation is clear: to move into the stronghold of the Taleban so rapidly and in such force that the enemy has no time to resist, regroup or flee. Pakistan has agreed to move up large numbers of its own troops to the frontier to prevent Taleban fighters escaping to the refuge of tribal territories. The Afghan Army, deployed alongside the Americans, will make it clear to the villages of Helmand that this is not a brief incursion: the Americans will occupy the ground, hold it and ensure that the insurgents do not return. US commanders will then begin the massive task of rebuilding the infrastructure, the economy and political stability. The troops will become familiar figures in the streets and markets. They will, symbolically, walk to work.

The influence of General David Petraeus is already clear. This is the same strategy that made the US surge in Iraq so effective. In Afghanistan, even more than in Iraq, what matters is the conviction among the local population that the allied forces are determined, better equipped and there for the duration. Afghans have for too long experienced three possibilities: occupation and control by foreign forces, or by the Taleban or — worst of all — temporarily by the allies followed by a vicious return of the Taleban. Their age-old response has been to offer allegiance and support either to the side that is decisively stronger or to the one that pays the more money to the tribes.

The US operation is what Britain and other Nato allies have been attempting for several years with marked lack of success. The reason is that it was always too little, too late. Britain’s forces were overstretched and underresourced. Early achievements, such as the capture of Musa Qala, were swiftly vitiated when the British were forced to withdraw. Yesterday, they were finally able to play a forward role, but suffered their most significant casualty, with the death of the commanding officer of the Welsh Guards. This only underlines the short-sightedness of the Treasury, which has refused to make the resources available for military success. It explains the bitterness of British commanders who have seen bravery and initiative thwarted by No 10’s refusal to authorise the troop levels needed.

The Taleban were caught on the wrong foot by the wave after wave of US helicopters landing behind their lines. Fighters entrenched for years in the valley melted away into the mountains. But the continuing threat should not be underestimated. The usual Afghan tactic is to withdraw, lie low and gradually reinfiltrate the villages. The US must now be robust in protecting those who want to register and vote in the forthcoming presidential election. It must move to deny traffickers and warlords the profits from the opium crop. And it must ensure that when US troop levels reach the planned 68,000, the initiative remains clearly with the allies. Victory is far from assured. But without bold action it was hopeless.