It's a simple question. Is the glass of Libyan rebel victory half empty or half full? And the answer, of course, is equally simple. Perhaps this cup doesn't run over, but it is full enough for everyone involved – except Colonel Gaddafi.

Take some of the supposed verities asserted over the six months since the revolutionary euphoria from Tunis and Cairo reached Benghazi. That air power alone could never tip the balance: that Britain and France couldn't foot the bombing bill for long: that this was a civil war doomed to stalemate and a country carved haplessly in two: that Nato's intervention guaranteed hostility across too many Arab streets: that it was yet one more pending disaster in the Iraq or Afghan pattern. Then take another sip from this simple cup.

The motives of Cameron and Sarkozy, as they first ordered their planes into action, seemed more humanitarian and emotional than cynically calculated. There was no urgent reason in realpolitik to oust Gaddafi as winter passed. His last 10 years in power had been quieter than his first berserk three decades. Labour home secretaries spooned his soup and drank his wine. Tony Blair embraced him. Libya's oil contracts were not at issue (just as they aren't today). The survival of Gaddafi's regime may have been a moral affront, but it was one among many. No: what sent British jets across the Mediterranean was a perceived need to save lives.

Tunisia had risen and its dictator had fled. Egypt had risen and Mubarak was finished. Benghazi had risen and now Gaddafi's tanks and planes were preparing vengeance. Could those who had the means to stop that stand by and declare what would happen next none of their business? A crucial decision, with Obama on the back foot and too much bruised British opinion feeling twice bitten, thrice shy. There was, and is, no great political dividend to be reaped: just a clear downside with not much of an upside. But, at heart, it was the right thing to do – a judgment call. And the events of the past seven days underline as much.

What do we find inside Gaddafi's ransacked compounds and villas? The gold-coated bling of wild corruption. Inside his jails? Political prisoners enduring torture and neglect. Inside the boundaries of stronghold Tripoli? See how fast that all fell apart as the rebel advance quickened. Of course there are tough pockets of resistance still. This is messy, block-to-block warfare, with Sirte yet to fall. But Tripoli, en masse, feels much like the Benghazi that seized its own moment. It is glad that Gaddafi is gone. It wants to help create something better – and fit into a wider context.

We know what Hosni Mubarak thought must follow once he stepped down. His son should become president of Egypt. We know who Saddam Hussein intended should follow him. His two psychotic sons. We know that Assad Mark Two succeeded Assad Mark One in Syria. We know that Gaddafi was preparing his own deluded dynasty. The "Arab spring", in short, was not some sudden convulsion that could be pushed aside by the forces of conventional authority. It was a move for something better in a region whose ageing rulers offered only more of something worse (and al-Qaida had donned the mantle of change).

Democracy is not a narrow creed (like communism, fascism or Ba'athism). It is not a terrible swift sword out to conquer the world. Least of all – from Wall Street to Hackney high street – is it an answer to every woe. No magic ingredients here. What it does offer, though, what it aspires to provide in regions where cupidity and cruelty are the familiar orders of every day, is the chance of something better. It lets the people decide what they want from life, and how they will be governed. It gives the masses a voice.

That is the chance that Libya has now. Of course (for the cup doesn't brim over) many things could go terribly wrong. Think a lingering Gaddafi menace; think tribal tensions, clashing ambitions, anarchy in a land without the old brute forces of law and order. But think, too, of the opportunities that may be grasped.

Nato was able to intervene in Libya because that was tactically possible. A string of cities along an exposed coastal road, a vast, empty land mass beyond. Libya is the 17th biggest nation in the world, with a population smaller than that of Switzerland. It has oil and the possibility of riches that, properly shared, may make it a leader in African development. It is religiously (Sunni) homogenous. It has the potential to grow, and grow together.

None of this offers any guarantees. When you put your faith in democracy (AKA other people), you're quite likely to wind up short. But it does, at least, lay out the basics of what should come next and how the months after eventual victory should be spent.

If the National Transitional Council wants advice (just as it wanted strafing jets overhead or a few key western organisers on the ground) then that should naturally be forthcoming. So should every effort to unfreeze bank accounts, extend recognition, play diplomatic friend and aid supplier of first resort. But fundamentally this is a Libyan rebellion, led and planned by Libyans. It is their chance, not ours, to manipulate from behind the arras. Least of all is it a template for further "liberal interventionism" by powers who think that turning Baghdad or Kabul into models of imposed democratic "freedoms" will somehow secure a more pliant world.

No. If Libya doesn't build on what's been achieved, then that's Libya's tragedy, not ours. Nothing is certain about the Arab spring – especially while Egypt's army retains its grip on the levers of society in Pakistan army mode, or while Bashar al-Assad can kill thousands of his own citizens and stay on top of the Syrian heap. Remember that the Taliban government in Kabul was a popular one, by Afghan standards. Observe already how the undercurrents of dawning Egyptian freedom tug discomfortingly at Israel's footing when Jerusalem seeks retribution as usual.

Nothing can be taken for granted here, nothing blindly celebrated. But suppose – no remote stretch of imagination – that Benghazi's ad hoc militias had been buried beneath the rubble of their hopes. Suppose that, when rhetorical push came to shove, the old champions of freedom had done nothing. Suppose that Saif Gaddafi was already anticipating his first days as bling supremo. Suppose that the cup was almost empty. It's infernally difficult, of course: but it's also quite simple, too.