Few experiences are more frustrating than seeing misery unfold before you as you stand helplessly by. Few provoke a stronger urge to cry: “Something must be done!” Add a cartoon baddie with a creepy Hitler tache, the ruination of a beautiful land and a televisually awful cholera outbreak and the cries for action get shriller still: “Send in the troops!”

The ruination of Zimbabwe provokes - in Britain, at least - many more such calls than most of the other miseries unfolding in Africa. Ghastly, intractable problems such as Congo and Darfur are not our problem. Our history as the former colonial power makes Zimbabwe our cause - and our refusal to intervene, moral cowardice, dressed up in historical excuses and lingering white guilt.

It is only 11 years since Clare Short, then Secretary of State for International Development, insisted otherwise, writing the maddest letter in the history of modern British diplomacy to Robert Mugabe. She repudiated Britain's “special responsibility” to fund land reforms, as promised in 1979. “We are a new government from diverse backgrounds without links to former colonial interests,” she opined. “My own origins are Irish and, as you know, we were colonised, not colonisers.” That's settled then, isn't it? International agreements be damned, my people died of potato hunger. So there.

To be fair to Short, Mugabe was fleecing Britain blind, and the situation could not go on. But Britain did have a responsibility to Zimbabwe, as the original architect of its unequal land distribution, to see justice done.

What followed is better remembered in Britain: the land invasions that began in 2000, which drove hundreds of white farmers and their families violently from their land. Britain screamed loudly - many of the victims were British citizens - and a new dynamic of antipathy between Britain and Zimbabwe was born. Britain was the evil imperialist seeking to recolonise “Rhodesia”; Mugabe became the “black Hitler”.

It is hard to describe how bizarre our Zimbabwe obsession looks from other vantage points. This weekend I dined with two friends, experienced foreign correspondents with a half-century of war reporting between them. Neither is British; both expressed bafflement at the British media obsession with Zimbabwe, a country both know well. Both have also witnessed the horrors of Congo, Rwanda, Darfur and Somalia.

How, they asked me, can people here seriously be debating forcible regime change in Zimbabwe while the millions killed by war and hunger in Congo and Darfur are met with apathy? Zimbabwe's situation is indeed appalling: a cholera epidemic that has killed hundreds, a collapsing economy, political terror and widespread hunger. It is also, we were forced to agree, the only story any of us had ever covered that is less awful on the ground than in the news. Congo is where I have heard the most stomach-churning stories of violence in my life. The testimony I have heard from Darfuri refugees convince me that they are victims of a genocide - and how often do we hear of our colonial legacy there? Yet Zimbabwe is the story that has this country angriest.

Military intervention is not going to happen, and Mugabe knows it. But not only would he love us to threaten it, he is already pretending that we have. The Zimbabwean Opposition is firmly against even the talk of military intervention, believing it drives Mugabe's military cabal tighter around him. Yesterday Mugabe accused Botswana, his nearest critic, of training insurgents against him, just days after it threatened to close its border with Zimbabwe. If we threaten it, we had better be seconds away from meaning it. Mugabe has shown us plenty of time already what he will do with his back to the wall.

We need to accept that we have no leverage with Harare and turn to those who do. South Africa is the regional superpower and the country on which Zimbabwe most depends.

But if we want South African leaders to act, we cannot make it suicidal for them to do so. South Africa is riven with its own racial problems and violence hanging over from its incomplete post-apartheid reconciliation. Hundreds of white farmers have been murdered in South Africa in recent years. That the killings are not part of an orchestrated campaign does not diminish the ferocious racial tensions they reveal.

No South African leader - not even Jacob Zuma, in whom we are investing a terrifying degree of trust - is going to unseat a black liberation hero at the bidding of a white former colonial power. So it is behind closed doors that South Africa must be told that while we understand its own difficulties, enough is enough.

South African eyes are focused on 2010, when footballing nations will gather for the World Cup. South Africa is already justifiably worried that its internal troubles may imperil the tournament. There are background whispers, still quiet, of a boycott.

Now is the time to show South Africa a yellow card. South Africa has already suggested that some teams could be based in neighbouring countries such as Namibia or Botswana, making it a genuinely African contest. Let's go farther still. Persuading footballing nations to send teams only to those countries, and play their matches there too, would deliver South Africa a monumentally embarrassing rebuke that no one could plausibly portray as racist or anti-African. Would the prospect of losing the World Cup stir South Africa into more action? We owe it to Zimbabwe to find out.

- Catherine Philp is diplomatic correspondent of The Times

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