So the general has departed. Saved by his senility, Augusto Pinochet will soon be reunited with the treasured collection of Napoleonic memorabilia at his Santiago home. His supporters are already toasting their leader's liberty. From Tony Blair's government comes an audible gasp of relief: Britain has been spared the messy diplomatic embarrassment of seeing the former Chilean dictator die in its custody. The big losers in this sorry saga are the victims of Mr Pinochet's crimes.

That anyway is how it seemed on Thursday as the aged dictator boarded a military plane for the flight back to Chile. To the relatives of the disappeared - the men and women battered and tortured before being consigned by the Pinochet regime to still-secret graves - justice had been elbowed aside by political expediency.

Yet the outcome is more complex. Much good has come from this episode. It may not be obvious to those grieving relatives. Nor to Margaret Thatcher, who declared the pursuit of justice a waste of British taxpayers' money. The gains are real nonetheless.

The Pinochet affair always had two dimensions. The first was the question - to which there was never a certain answer - of whether, however guilty, a visibly frail 84-year-old could be punished for the wickedness of his regime. The second centred on whether his detention in Britain at the request of a Spanish court would advance a much bigger cause: the entrenchment in national laws of the principle that some crimes are so heinous as to demolish the claims by their perpetrators to sovereign immunity. Looked at like this, his release takes on a different hue.

It is tempting to conclude that Jack Straw, the home secretary, was looking from the outset for a get-out from the extradition requests lodged by Spain and three other European nations. For the left in Europe, the torture and murder seen under Mr Pinochet during the 1970s and 1980s has always engendered a particular revulsion. Perhaps it is because Chile held a mirror to Europe's own encounters with fascism. Peter Mandelson, hardly a leftwing firebrand, characterised as "gut-wrenching" the notion that the general might escape under the cloak of diplomatic immunity.

Power, though, tends to drain politicians of their idealism. Though Mr Straw was careful always to play by the rules, the Blair government made little secret of its discomfort during the proceedings against Mr Pinochet. The Foreign Office - planning even now to despatch one of its ministers to smooth ruffled feathers in Santiago - didn't like the disruption of its Latin American diplomacy. The trade department fretted about the loss of lucrative export contracts. And the defence ministry has always put arms sales ahead of ethics.

Yet in the end, the medical reports that gave Mr Pinochet his freedom were a happy coincidence for the Blair government, rather than the product of a squalid conspiracy. A group of independent clinicians judged that the general was mentally unfit to face the rigours of a trial. To haul him through the courts - and the process would have taken several years - would have been to risk turning a monster into a martyr. Mr Straw was right. To conclude that the government was delighted to find this escape route is not to say it was a political fix.

Mr Pinochet, of course, will not face trial in Chile. For all the fanfare about that country's transition to democracy, it is too often forgotten that it has been on the military's terms. Mr Pinochet is protected by his own amnesty. As a senator-for-life, he has legal immunity. And even if that could somehow be overturned, he would answer before a tribunal of his long-time military cronies.

Yet, although this will be scant consolation to those who suffered the cruelties of his regime, Mr Pinochet returns to Chile with his evil crimes exposed. He is a pariah. He will not dare again to venture beyond his own country's borders. Mr Pinochet craved respectability in his old age. The ghastly descriptions of his regime heard in the British courts - of opponents routinely mutilated and tortured in the vilest manner imaginable - have torn off that thin veneer.

That takes us to the second dimension of this case. Mr Pinochet has been released, but the court rulings that kept him in Britain for more than 16 months have breathed legal life into the several international conventions that assert the primacy of certain basic human rights over claims of state sovereignty.

Sure, the British judges handed down rulings that sometimes seemed confusing and even contradictory. But, at their core, these judgments asserted a vital principle. Former dictators charged with crimes against humanity cannot any longer hide behind the doctrine of state immunity. If they venture into the democratic world, where the law operates independently of governments, they must expect to finds themselves hauled before the courts.

The principle, of course, was first established at the Nuremberg trials. It lies behind international conventions against genocide and torture, the second of which led Mr Pinochet into the British courts. It is spelt out in the terms of reference of the tribunals considering war crimes in the former Yugoslavia and in Rwanda. It is there too in the statutes of the International Criminal Court.

What's important about the Pinochet case is the demonstration that these international expressions of good intentions will be given teeth in national jurisdictions. Public officials, presidents or not, will be held to account for crimes against humanity.

Of course, it will be said that enforcement of this patchwork of supra-national laws is haphazard. And it is likely to remain so. Realpolitik will continue to play its part. Some of those guilty of the most heinous crimes will escape by virtue of their political or military power. We have seen that in the operation of the tribunal dealing with the former Yugoslavia. And the Vienna Convention still affords immunity to tyrants who remain in office.

There will be occasions too - though far fewer than has been claimed by the apologists for Mr Pinochet - when strict application of the law might be judged damaging to efforts to promote national reconciliation. There is sometimes a place, a small one, for flexibility and pragmatism.