Obama finally made his mind up. The US would intervene to stop him. And there would be no half measures. All steps short of boots on the ground, as the US under-secretary of state William Burns put it are now urgently contemplated, with a view to immediate implementation.

That means possible, imminent air strikes as well as an air exclusion zone. It means direct head-on combat with Libya's air force, if it chooses to fight. It means, potentially, western casualties, if pilots are shot down or bail out or are taken hostage. It could mean innocent civilian deaths as the EU's foreign policy chief Lady Ashton warned last week. And if things do not go well, it may mean escalation beyond all that is envisaged now. Who knows when it will stop.

The immediate impact may be to stop Gaddafi's advance on Benghazi in its tracks. If that happens, the revolution will have been salvaged, albeit at the very last moment. Whether it can endure is another matter entirely.

The US and its European and Arab allies will hope that Gaddafi, facing the prospect of overwhelming, punitive force, will quickly back down, observe the UN demand for a ceasefire, even agree to negotiations. But to be sure of saving Benghazi, a no-fly zone will not be enough. To drive home the point the game is up, it is likely allied air strikes on Gaddafi's heavy armour and artillery will be required, and possibly also attacks directed at him personally, as Ronald Reagan tried in 1986.

Gaddafi acts like a bully and a coward. But he is full of bluster. Only a sudden, bloody nose will convince him to desist. This is he is probably about to receive. And the betting must be that, once the revised odds become clear, those remnants of the Libyan army and security forces that have so far remained loyal will desert him, too.