It's a sad end to a fine career, but for the good of his country Mr Rumsfeld has to go

It is impossible to allow such an important post to be held by a damaged figure, struggling for political survival

One accused US female soldier is claiming that no one told her about the Geneva Convention. Gee. As a defence, this ranks just below the fellow who murdered his parents and then asked for leniency because he was an orphan. That soldier and her guilty colleagues have defouled and dishonoured a great army, a great nation and a noble cause.

Yet, albeit inadvertently, the girl was making an important point. She is not solely to blame for her appalling behaviour. Morality is more a matter of custom, habit and the constraints of circumstance than we care to acknowledge. When those all break down, Lord Of The Flies is never far away. By no means all the monsters who manned the atrocity services of the Third Reich were born monstrous. A lot of them slipped into evil because the higher-ups had authorised it and others were doing it. Thus evil itself became custom and habit; the banality of evil, as Hannah Arendt put it.

After all, there seems nothing inherently evil about private Lynndie England, who poses cheerfully in most of the worst photographs. She looks like a smiling, wholesome girl; the sort of kid who might serve lunch at Joe's Diner in any small American town, saying "Y'all have a nice day", and sounding as if she meant it. She cannot be absolved of all responsibility for her actions; she is too old for that. But it is not only her fault that she is now indelibly associated with shame and disgrace.

Where were the officers? All armies recruit from rough backgrounds. In the field, enlisted men are a long way from softening influences. They are exposed to discomfort and danger, and the average private soldier will feel little affinity with the Geneva Convention. That is why the British Army devotes so much time and effort to training and discipline - and to vigilance.

Officers are expected to know what is going on. The other month, I was having a late night drink with Giles Shephard, who was once in the Coldstream Guards. Since then, he has been a peerless hotelier at both The Savoy and The Ritz. Giles put down his glass, but not to go home. He was off back to The Ritz, for a tour of inspection. His staff never knew when he would turn up. They only knew that if he did, he would have Argus eyes. He learned in the Coldstream how a good regiment should be run, and a good hotel.

It may be, though this is still unproven, that some British soldiers have been guilty of excesses in Iraq. If so, these will have occurred in or around the urgency and chaos of the front line, where good order is hard to maintain. I do not believe that any British unit would be guilty of the systematic and prolonged abuse of captives. Under our system, that could not happen.

Under the American system, it did. Those who were responsible must be held to account, and that must include the Defence Secretary himself, Donald Rumsfeld.

Mr Rumsfeld cannot be held personally responsible for the initial offences; that would be absurd. But he was guilty of a lack of urgency in response. From the first moment, he should have realised that he was dealing with a public relations catastrophe and a moral crisis - not to mention an electoral embarrassment. He should have instantly dispatched a tough commander to impose a grip on the prison, sack or arrest the malefactors, and have a report on the Defence Secretary's desk within a week.

While offering his resignation, Secretary Rumsfeld could then have alerted the President. He could also have levelled with Congress and prepared a media strategy. There would still have been outrage - as there ought to have been - but at least the administration would have appeared to be in control of events. Bush could then have brushed aside the resignation letter, pointing out that Rumsfeld had done everything possible. As it is, however, that is not true. So he ought to go.

In Washington, the word among his friends is that it would be unfair if Rummy's career ended in this way. That is true. He has been an outstanding public servant and deserves a better exit. But his friends cannot have read the politicians' Geneva Convention. It states that politics is never fair. It also includes Enoch Powell's over-quoted but indispensable dictum: "All political careers end in failure."

Mr Rumsfeld has a further problem. He does not have that many friends. It is his misfortune that there has never been a Pulitzer prize for abrasiveness. A friend of mine once said that you could go into a meeting with Don Rumsfeld on his side, and 30 seconds later he would have one hand around your throat and the other round your balls. That was when you were on his side. Over the years, he has been in a lot of meetings with people who were not on his side. He would have enjoyed himself. They would not.

That is why there is a lot of scarcely concealed gloating about his recent misfortunes, especially among the senior military and the State Department. The relations between Mr Rumsfeld and his deputy, Paul Wolfowitz, and Colin Powell and his deputy, Richard Armitage, make Tony Blair and Gordon Brown seem like David and Jonathan.

All this will make it much harder for Mr Rumsfeld to rebuild his authority. That is a further reason for him to resign. At a crucial moment, it is impossible to allow such an important post to be held by a damaged figure, struggling for political survival. This might not be the career ending which Donald Rumsfeld would have chosen. But it may still be the nearest that he could come to a dignified departure.

His successor might then have a chance to refocus the world's attention on the mission in Iraq, rather than on the unnecessary crimes which have been committed in its name. Despite recent events, the case for invading Iraq remains as strong as ever. Saddam Hussein ruled a rich country with an able population. Yet he was bankrupting it and brutalising them while trying to acquire terrible weapons which would have destabilised the region, and the whole world. All roads to progress in the Middle East lead through Baghdad.

The Americans are not in Iraq to maltreat prisoners and steal oil. They are trying to enable Iraqis to govern themselves, and to stick to a timetable to hand over power. In 10 years' time, if the US has its way, Iraqis will be freer and richer than ever before. A well run, prosperous Iraq could also be an example to the rest of the Arab world. That is why the Abu Ghraib jail is such a disaster. A prisoner is dragged on all fours, like a dog. In so doing, his captors appear to put America on all fours with Saddam Hussein. US soldiers went into battle to close down the torture chambers. Now, the unspeakable behaviour of a handful of Americans has enabled the West's enemies to claim that the US's regime in Iraq is no better than the one it replaced.

This is all nonsense, and malicious nonsense at that. But it cannot be properly refuted as long as Donald Rumsfeld is in office.