The landscape after the battle, in a conquered country, does not smile in a warm morning of freedom. Instead, there begins a rat-infested twilight, and many of the rats are human. The prisoners will emerge and the exiles will return. But as they shoulder their rucksacks and try to find their homes in ruined streets, they will often see those who imprisoned and exiled them riding past in the conquerors' jeeps, wearing new armbands of authority. Politicians in new offices will sell options on good jobs and stolen aid shipments. Decent families will scrabble like white mice for food and favours.
Iraq, at first, will be no different. But the world cannot afford to leave it like that. For this potentially wealthy country of 23 million people, with a large and sophisticated middle class, there has to be a new invention of nationhood. The sad limbo status of yet another UN protectorate, partitioned and mafia-ridden, is not an option for Iraq. With neighbours like Iran and Turkey, the appearance of an enormous grey area of indefinite sovereignty in one of the most contested regions on earth would invite catastrophe.
Incredibly, with American tanks half way to Baghdad, there is still no agreement on how to run a military occupation regime, let alone on a programme to reconstruct an Iraqi state. (The best suggestion so far is for a UN "blue police force" drawn from Muslim countries to restore order and justice at local level.) But last week's quarrel at Brussels is not as serious as it looks: Tony Blair is evasive about free elections in Iraq, but at least he and Chirac seem to agree that the security council must authorise a post-Saddam civil authority. The real trouble is in Washington.
There, the most extreme hawks not only reject American involvement in "nation-building" but resist any role beyond emergency aid provision for the detested United Nations. They are likely to be overruled. Jay Garner, the retired American general who is supposed to become the temporary civilian head of the occupation authority, knows that the UN will have to take political responsibility of some kind, and last week's Azores meeting committed the reluctant President Bush to seek security council endorsement of "a post-conflict administration". But precious time is being wasted.
The project of building a strong, just and reasonable Iraq faces awful obstacles, but starts with two huge advantages. The first is the sheer speed of the American-British onslaught. This means that there has been no time for regional warlords to get their armed forces into the act as recognised "allies" and claim a share of central power. And the speed of the advance may also - with luck - ward off the real doomsday scenario now looming over the conflict. This is a full-scale, Cyprus-style Turkish invasion of northern Iraq, which would crush the Kurds, cripple a future Iraqi state and destabilise the whole Middle East for a generation. If the Americans can get first and in force to Mosul and Kirkuk, they may be able to head off this disaster.
The second advantage is the powerful tradition of Iraqi nationalism. All nation-states are constructs, and the fact that Iraq was invented by the British in 1920 out of three Ottoman provinces has not prevented the growth of a patriotism directed largely against foreign interference. The British granted Iraq formal independence in 1932, but returned heavily during the second world war and pushed Iraq around for cold war purposes until their credibility collapsed after Suez. Two rebellions against western "neo-colonialism" have become mythic. The first was the unsuccessful 1941 revolt against the British by Rashid Ali, misleadingly dismissed by western historians as "pro-German". The second was the savage putsch by General Qasim in 1958, which murdered the king and tore Iraq out of the pro-western Baghdad pact. The ensuing struggles, which ended in Saddam's dictatorship and the one-party rule of the Ba'ath, have not diminished Iraqi pride in an independence perceived as wrested from foreigners by force. And this tradition, although hijacked and betrayed by Saddam, is still solid enough to build a new state on.
What sort of state? The example of postwar Germany suggests that the best ideology for the purpose is social democracy. One of the first things the British did in their zone of Germany was to sponsor a new trade union confederation, the sheet anchor of democracy in the years to come. But this approach is now unthinkable. So is any "Mesopotamian Marshall plan". Instead, Iraq will probably be abandoned to the joys of an uncontrolled free-market regime, supervised by the World Bank.
Iraq owes foreign financiers some $200bn to $400bn in debt. If the experience of Serbia after its own "regime change" is anything to go by, almost all the financial aid offered by the "international community" will be clawed back into debt repayment. Iraq's oil revenues of some $10bn a year will probably go on being managed by the UN oil-for-food programme. The Iraqis, in other words, will be generously permitted to go on paying for their own food and medicine. Moreover, the Saddam regime was maintained not only by terror but by an enormous network of kinship-based corruption. The tale of post-communist Europe suggests that if a one-party controlled economy is instantly opened to unregulated capitalism, party networks of clientship turn rapidly and naturally into relationships of organised crime.
But the most urgent question is the state's form. Put crudely, what sort of constitution can prevent ethnic and religious civil war, if Shia and Sunni Muslims and the Kurds demand autonomy or independence and resort to arms? For outsiders, the obvious answer is a loosely federal constitution. But things are not so simple. Many Iraqis fear that autonomy would lead to disintegration; the Kurds pushing for full independence and the Shia falling under Iranian influence. Just possibly, the opposition parties now in exile could agree to a federal deal. The danger is that a large part of the Iraqi people might reject such a settlement as a betrayal of national unity.
Then there is the question of Islam. Iraq, under the parliamentary democracies before 1958 as under the Ba'athist dictatorship, has been a secular state. But the Americans, above all, have to accept that this is going to change. Islam is going to be much more powerful in the new Iraq, and not only in the Shia south. If the transitional governors show wisdom, a moderate form of sharia law can co-exist with liberal democracy. If they panic, then a surge towards fundamentalist theocracy could become unstoppable. And the Americans will also have to accept that a free, democratic Iraq will support the Palestinian cause and condemn Israel.
Liberation hurts. In Iraq, it comes with humiliation and fear about the future. A UN transition regime must replace the military governors as soon as possible, and must move quickly towards democracy. And the White House fanatics have to realise that a free Iraq cannot be designed to suit their ideology. It will be ungrateful. It will have policies they dislike. This is called independence. If it is denied, then the real liberation of Iraq will happen unpredictably, and bloodily, in the future.
· Neal Ascherson is the author of Black Sea (Vintage) and Stone Voices (Granta)