Leading article: Washington finally realises the role it must play in saving Libya

Slowly but surely, the US is being propelled back into the driving seat in the West's war on Muammar Gaddafi.

The US defence Secretary, Robert Gates, talks only of making a new, "modest contribution" to Nato's air campaign against the regime but the humble phraseology should fool no one. America's decision to deploy Predator drones to shore up Libyan rebel positions is a critical development in the conflict – marking the moment when Washington accepted that dislodging Gaddafi would require more US firepower.

Only a few weeks ago, on 4 April, Washington handed the lead role in the campaign to Nato, in effect consigning the business of Gaddafi's overthrow to Britain and France. Since then, the rebels have stalled; key "swing" tribes in Libya have thrown in their lot with Gaddafi; the regime has grown more confident, and Gaddafi forces have given the western rebel enclave of Misrata a battering. Without increased US military aid, the rebels appear condemned to remain in control only of Benghazi, a slab of territory in the east, and possibly a fragment of Misrata. At worst, the western enclave could fall, providing Gaddafi with a propaganda victory as well as enabling him to release troops for the eastern front.

While the prospect of the US assuming a more active role in the conflict will be a great relief to David Cameron and Nicolas Sarkozy – both now surely regretting earlier, confident talk of regime change – President Barack Obama will be full of misgivings.

A handful of senior US politicians, such as the Republican Senator John McCain, who visited Benghazi yesterday, are firm advocates of Western intervention to overthrow Gaddafi. Otherwise, the Libya issue has little traction with Republicans or Democrats, or with the public in general. In fact, Libya was never seen as especially strategically important, which is why no one objected to Italy – the least important great power at the turn of the 20th century – occupying it just before the First World War. As an item on Washington's foreign policy priority list, Libya ranks far below Iran, Saudi Arabia, Egypt, Yemen – and Bahrain, for that matter.

Nevertheless, Mr Obama has reluctantly concluded that he cannot afford to let Nato be humiliated in Libya, and that if America does not take a more active role in trying to shape the country's future, the danger is that it will slide into the abyss. Other considerations add to a sense of urgency among the Western policy-makers on Libya. The conflict has already prompted a quarter of a million people to flee across the border, mainly to Tunisia. The nightmare scenario for countries on the other side of the Mediterranean is that the northward flow of asylum-seekers from the region, which has increased recently, will turn into a flood if the conflict drags on interminably and life in Libya becomes intolerable.

The hope must be that US drones speed up the process of degrading Gaddafi's military capacity, relieve Misrata and force the authorities in Tripoli to get serious about negotiating an end to the conflict – one of the ingredients of a settlement being Gaddafi's departure. The concern in Washington will be that the US is being sucked into a war in which no end is in sight and in which it has no real stake – and that each new "modest contribution" only fuels calls for it to do more.