Rupert Cornwell: At the moment of truth, the rule of a tyrant rings hollow

Has there been anything like it since the end of the Second World War, when the Allies invaded and defeated Germany and Japan and imposed entire new systems?

True, in the past 20 years, US troops have gone into Grenada and Panama to impose regime change by military force. Never though has the feat been accomplished, apparently successfully, as with the Anglo-American invasion of Iraq and the destruction of Saddam Hussein's regime? Dictatorships and authoritarian governments die in different ways. Some collapse in violence. Others perish in slow motion – like the Soviet Union when perestroika undermined the system from within. Soviet Communism truly died with the attempted anti-Gorbachev putsch of 18 August 1991, when the would-be restorers of Leninist orthodoxy failed in that most basic of communist skills, of organising a decent coup.

A few months later Russia, Belorussia, and the Ukraine pulled out of the Soviet Union, and when the Russian flag replaced the hammer and sickle above the Kremlin on Christmas Day that year, and President Mikhail Gorbachev resigned, it was the quiet sealing of the self-evident.

Invariably, however, these regimes collapse as a result of internal pressures from below not, as in the case of Iraq, with an almighty push from outside. Only perhaps in the birth of Bangladesh, when India's army played the decisive role in the disappearance of the former East Pakistan in 1971, did something similar occur. By the time the Indian army rolled in, the East was already in rebellion against the dominant West Pakistan.

But for the rest, whether peaceable or violent, it has been the people that have sparked the revolution, not infrequently with the crucial backing of the military, to overpower the secret police who usually are loyal to the tyrant to the last. Even Vietnam 1975, portrayed as the ultimate American defeat, was the culmination of a 20-year internal war of national liberation and re-unification.

So it was in April 1974 in Portugal, at least until the uprisings in Eastern Europe in 1989 the most memorably joyous and peaceful of them all. "Um povo unido jamais sera vencido," the people of Lisbon chanted, "a united people will never be defeated," as they thrust red carnations in the barrels of rifles carried by the soldiers whose swift uprising blew away the stifling Salazar/Caetano dictatorship like dust from the palm of a hand.

Later that year the Portuguese example was followed by Greece; only this time it was the vicious and ignorant military junta of the colonels that crumbled amid the humiliation of the botched coup in Cyprus. Within hours, Constantine Karamanlis, the former prime minister "dans la reserve de la republique," returned from de Gaulle-like self exile in Paris, and thousands of Greeks rushed to the airport in the small hours of the morning to greet him in joy.

Five years later, another exile named Ayatollah Khomeini returned to his native Iran on the morning of 1 February 1979 to an even more tumultuous welcome, as a million people waited at Mehrabad Airport, and two million more who lined the route into Teheran. A fortnight earlier the Shah had slunk off to Egypt for what was officially called an "extended vacation," but which everyone knew was for good.

An eyewitness to Khomeini's return wrote of the extraordinary atmosphere it generated, of how "a millennial frenzy took over the country.

Pace Ahmed Chalabi Iraq has no Karamanlis and (perhaps just as well) no Khomeini. As it realises that this time Saddam's downfall is for real, that country too may be seized by a "millennial frenzy" of jubilation and hope.

Outwardly, the scenes yesterday in Baghdad – the sporadic fighting, the looting and the toppling of the dictator's statues – recalled nothing so much as Bucharest 1989, when a little noticed protest in the northern Romanian city of Timisoara in mid-December suddenly metamorphosed into a national uprising that saw the megalomaniac Nicolae Ceaucescu and his wife driven from power and executed after a summary court martial on Christmas Day.

But in Iraq, it took an almighty shove from outside to achieve what in 1991 even external defeat in Kuwait and internal uprisings in the north and south could not. The circumstances of Saddam's fall mean this will be nationbuilding on an epic scale.

Germany and Japan were after all homogeneous, developed societies cowed by shame at what their governments had done in their name. That is one reason why it will be a miracle if post-Saddam Iraq matches Portugal, the Czech Republic, Poland and the rest, and attains the functioning democracy of which Washington's theorists dream.