Just as the rhetoric from the White House, Downing Street and Brussels was reaching a new peak of righteous outrage, opposition leader Morgan Tsvangirai published a heartfelt plea in a number of western newspapers. It contained this phrase: "The people of Zimbabwe need the words of indignation from global leaders to be backed by the moral rectitude of military force."

Military force? Ah. The governments of the West stopped in mid-denunciation. Tumbleweed blew across the floor of the United Nations Security Council meeting room in New York. Is the UN or Nato really prepared to come to Tsvangirai's aid and use military force to help oust Mugabe? Not a chance.

The request for help is couched in what seems like reasonable terms: "We do not want armed conflict… (but] we need a force to protect the people. Such a force would be in the role of peacekeepers, not troublemakers. They would separate the people from their oppressors and cast the protective shield around the democratic process for which Zimbabwe yearns."

Tsvangirai is calling the West's bluff. Given the rhetoric of recent years from Washington and London about the need to depose despots and bring democracy to benighted corners of the world, the message coming from the Zimbabwean opposition is a simple one: prove to us that these are not empty words; prove to us that the West cares about these principles even when there is no oil at stake; prove to us that you are not hypocrites.

What Tsvangirai suggests is a peace-keeping force, but, of course, it wouldn't be as simple as that. Any western military force deployed by the UN or Nato would be taking one side in what would quickly become a civil war. It wouldn't be a straightforward question of isolating Mugabe and a few henchmen.

Inevitably, any western intervention would justify the Mugabe propaganda that opposition to his rule is the residue of British colonialism and a symptom of the racist view that the white man knows best. Most pertinently, while Mugabe and his generals control the army, force would be met with force, bringing about a bloody conflict on a scale as yet unseen by Zimbabweans.

Last week Paddy Ashdown suggested an African Union force, with western support, could intervene if the violence worsened. But African leaders show no sign of having the appetite for such an operation.

For people like myself who supported military intervention in Kosovo, Afghanistan, and (having been misled about the evidence on WMDs) Iraq, Zimbabwe poses a moral dilemma. I believe it is sometimes justified to breach national sovereignty. There's no doubt in my mind that the international community should have intervened militarily in Rwanda in 1994. So why not Zimbabwe in 2008? The only answer is cruel and calculating – but unavoidable. In Rwanda, what the world was faced with was genocide. In Zimbabwe, the killing has been widespread and appalling, but not nearly on a Rwandan scale.

Back in 1981 at some international conference or other, Mugabe was chatting to Winnie Ewing, the grande dame of Scottish Nationalism. She complained to him that the SNP was not doing too well in whipping up nationalist fervour. Mugabe had some words of wisdom to impart. "That's because the people of Scotland are not yet sufficiently oppressed," he said.

After so many murders, rapes, incarcerations and beatings, it seems a cold judgment to say that the people of Zimbabwe are not at this moment "sufficiently oppressed" to justify a military invasion to secure and enforce democracy. Yet this is the international community's verdict on Tsvangirai's heartfelt appeal. It's hard to argue otherwise.

So what is to be done? Last week brought the first encouraging signs that cracks were appearing in the Harare regime. There were reports that senior colleagues of Mugabe were making overtures to the opposition about doing deals to save them from persecution if a new government took over. Could the regime collapse in on itself, taking Mugabe with it?

The West takes a gamble every time it hopes that the internal dynamics of a troubled country will result in its despotic leader being deposed. Sometimes this succeeds – Ceausescu in Romania and Milosevic in Serbia were both ultimately brought down by their own peoples. But more often this is a gamble that fails, or goes awry. When Nato forces expelled Saddam Hussein from Kuwait in 1991, hopes that the Iraqi opposition could be relied upon to topple him proved overly optimistic. They underestimated the brutality with which Saddam could identify and punish dissent. Worryingly, in Zimbabwe last week, Mugabe's security forces were trying to identify which members of the regime had been putting out feelers to Tsvangirai.

What of the other options? It's hard to endorse the sanctions regime currently being demanded by all sides. True, there is some satisfaction to be gained from depriving the regime's senior figures of the right to international travel, foreign bank accounts and a British education for their children. But the experience of Iraq is that broader economic sanctions only serve to further impoverish the already pitifully poor. Zimbabwe demands our attention. But history tells us to take care in responding to the cry that "something must be done!"