With a boldness that the world had begun to believe he lacked, Barack Obama has gone for broke. The US wants Muammar Gaddafi's head. It will not rest until he is deposed and there is regime change in Libya. And it will fight to get it.

Obama spent weeks pondering, prevaricating and posturing, infuriating Britain and France, arch advocates of military intervention. He used public appearances to prate professorially about plans, contingencies and downsides. He allowed senior administration officials such as Pentagon chief Robert Gates to give full vent to their doubts and misgivings about a possible Libyan quagmire.

Obama is already fighting two wars in Muslim countries he did not start – in Iraq, now all but finished, and Afghanistan. He did not want to author another. He did not want another foreign distraction ahead of his presidential re-election bid next year. He did not want the cost, the corpses or the inevitable collateral damage – political and human.

But gradually the pressure from hawkish Democrats such as John Kerry and Republicans such as John McCain began to tell. The escalating rhetoric from Downing Street and the Elysee Palace will have had an impact, too.

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.

The longer term impact of the intervention is immeasurable – but disaster is certainly one possible outcome. Like the first Gulf war, the involvement and support of Arab countries means the Libyan war will not be defined, except by hardline jihadis and al-Qaida, as another western assault on Muslim lands. But if the fighting is prolonged, if Gaddafi does not quit and run, if his more able sons take up his cause, if the intervention makes things worse not better for ordinary people (as in Iraq), if there is no clear-cut win but ongoing low level conflict and resistance (as in Afghanistan), then Arab opinion will turn against the westerners once more. The post-9/11 nightmare of the Pentagon's long war without end will reproduce on the shores of the Mediterranean.

But there is a reasonable prospect of success, too. If the rebels, rescued from annihilation, prove capable of creating a government able to take over the running of all of Libya, and not just the rebellious east, then Obama's gamble could pay off.

If Gaddafi, no longer able to deploy superior firepower and mercenaries, is overthrown by his own people, it will be hailed as an improbable triumph for, among others, David Cameron, who took a harder line than most, earlier than most. Britain (and not Germany, which opposed intervention) may profit from the gratitude of a grateful people. If Libya falls to democracy, then like-minded reformers in Bahrain and elsewhere will be greatly heartened.

Obama and Cameron are looking for another Kosovo or Kuwait, not another Iraq. It's a story, as they would prefer to write it, with a happy ending, producing a newly independent country, and another friend for the west. But they cannot control the outcome. Now they can only wait and hope they were right.