Robert Mugabe's Zimbabwe is a joke. Not a good joke, and certainly not a funny joke. Yet had you set out to caricature a dictatorship and satirise the logic of all tyranny, tracing every disaster from hopeful beginnings to squalid conclusions, the man and the country he controls would fit the bill. Where Mugabe has ruled, everything that could go wrong has gone wrong. Here there is tragedy and farce in one.

He was a hero once. To some, he still is. He seemed, better than most, to embody a tradition of African nationalism in the struggle with white colonialism. To hear him tell it, he is still fighting that good fight, and still calling Britain to account for its colonial past and its presumptuous present. After a quarter of a century and more, however, the old man is running out of convenient villains.

The west's post-colonial record in Africa is nothing - at best nothing much - to be proud of. Neglect, influence-peddling, political interference, debt bondage, arms sales and infinitely patronising attitudes have featured heavily. Talk of shared histories and responsibilities have been cheap. When it has mattered most - in Rwanda, in Darfur - the contrast between reality and rhetoric has been stark. Mugabe's singular achievement, where Zimbabwe is concerned, has been to erase much of this from the memory.

British rule in the old Rhodesia did its African people no favours. Ian Smith's UDI state stripped away the mask of benign colonialism for good. The theft of land - Mugabe's justification ever since - was wholesale and blatant, the racism undisguised. In 1981, independence and liberation were held to be one and the same thing: Bob Marley even wrote a song, a good one, to that effect. And since? Here the joke becomes elaborate, like a bleakly comical guessing game. It would be easier to ask what is right with Zimbabwe than to count the ways in which things have gone terribly wrong. A complete list would fill several pages.

Democracy? In order to ensure his own rule Mugabe has rigged elections time and again. Not even his friends have been able to blame that on nefarious Britain. Opponents have been assaulted, jailed or killed; journalists are censored ruthlessly; and, daily, the people flee. Some say that one-third have gone to South Africa and elsewhere.

The economy? In the crudest sense, that doesn't exist. The official best guess puts inflation at 7600%, but that is presumed to be an underestimate. Some 80% of the remaining population live below the poverty line. The black market, where £1 sterling is worth 500,000 Zimbabwean dollars, is the single growth industry. Agriculture, once the country's pride, has never recovered from Mugabe's decision at the turn of the century to reclaim land from white farmers and redistribute it, often to cronies. The Aids crisis has hit Zimbabwe hard. One adult in six under the age of 45 is thought to be affected.

Shops have been emptied since Mugabe decided to fix inflation with crude price controls. One household in five has no access to decent water. Petrol is scarce and power cuts are frequent. Four million depend on food aid (Britain is the second-largest donor) and life expectancy, having fallen year by year, stands at 37.

Habitually, Mugabe blames British "sabotage". Some African states such as Angola, protective of their independence and sensitive to outside interference, believe him. An old British promise of "reparations" for land thefts is still invoked and is still, in some quarters, effective. It was striking, meanwhile, that the Zimbabwean leader was met with loud applause when he turned up at a meeting of southern African presidents earlier in the year. For some, even 26 years on, the aura of the liberator remains.

So what? Britain's recent experiences in the great game of liberal intervention have not been distinguished. We pick and choose our battles, often cynically. We can acknowledge our colonialist debts with aid and fine words, but if yet another African country is at the edge of destruction thanks to yet another dictator, what are we supposed to do. Invade? Ignore the mad accusations? Exert diplomatic pressure, whatever it might be worth, on South Africa's Thabo Mbeki to make some sort of progress after years of pretending to broker a deal between Mugabe and the (divided) opposition Movement for Democratic Change?

Zimbabwe is a test case. It can tell us one of three things. Either the western democracies mean what they say when they fret over the condition of Africa, or they are hypocrites and do not truly care, or - the answer no-one really wants to hear - there is nothing worthwhile to be done. In the last case we do our best to keep the World Food Programme going, hope that Mugabe falls from his perch soon, and ensure that he is given no opportunity to sit in the same room with a British Prime Minister.

That, so he says, is to be Gordon Brown's gesture. He wants to toughen up a modest sanctions regime. In the meantime, he will refuse to attend a meeting between EU leaders and the African Union in Portugal in December if Mugabe is allowed to be present. The point, as the Prime Minister wrote in a London newspaper this week, would be to prevent the dictator from "diverting attention from the important issues that need to be resolved".

Zimbabwe is important. After Darfur and continuing havoc in the Congo Republic, it probably counts as the most important African issue. As he indicated when visiting the continent shortly before becoming Prime Minister, Mr Brown cares deeply about Africa's fate. But you have to ask: is this it? His disgust at the thought of encountering Mugabe shows principle. What will it change, precisely?

The International Crisis Group reports that Zimbabwe is "closer than ever to complete collapse", yet advises Britain not to interfere. Desmond Tutu and John Sentamu, Archbishop of York, take precisely the opposite view. Last weekend the latter wrote: "The time has come for Mr Brown finally to slay the ghosts of Britain's colonialist past by thoroughly revising foreign policy towards Zimbabwe and to lead the way in co-ordinating an international response."

The archbishop, Ugandan born, added that the "time for African solutions' alone is now over". Britain, he said, must mount the kind of campaign that was mounted against the Smith regime and against apartheid South Africa. Sentamu demanded "smart", targeted sanctions against the Mugabe elite. "We cannot look the other way on Zimbabwe," he said.

True, I think. Certainly it would offer a more honourable cause than anything Iraq has provided. Yet even given Mugabe's disgusting record, the implicit irony of intervention is striking. Are the western powers truly to return to Africa in the 21st century determined to civilise the natives? If so, we had better ask again why our failures were quite so catastrophic in the first place.

Was it all our blame, as a self-serving Mugabe would have it, or none of our fault, as we prefer to believe? Somewhere between the two, I suspect. Meanwhile, the pragmatic Chinese, busy befriending governments all over Africa, have just deserted their client in Zimbabwe. No more subsidies from that source. Liberal intervention in Mugabe's wasteland, when and if it comes, may amount to no more than an effort to pick up the pieces.