The world is right to be cautious. It has taken seven weeks of talks, with frequent interruptions and breakdowns, for Morgan Tsvangirai, the Zimbabwean opposition leader, to reach an accommodation with President Mugabe. Even now, the agreement apparently reached on Thursday is far from clear. It will not be until Monday, when the two sides officially sign a deal to share power in a new government, that the details are made public. Those details are crucial. For this agreement is either a long-awaited breakthrough, the first step towards ending the Mugabe dictatorship and reviving the shattered country, or it is yet another ruse by the 84-year-old autocrat to hang on to power and escape retribution for years of tyranny.

Mr Tsvangirai's supporters are unhappy still with some of the agreement's provisions. That is hardly surprising. The latest compromise on the key issue of where power rests proposes to make Mr Tsvangirai Prime Minister with responsibility for the Cabinet. At the same time, a new council of state under Mr Mugabe would be created, which will oversee the Cabinet. The President, who would be joined by two of his deputies, would however have no veto on the council. This looks like a recipe for confusion and deadlock - or else a loophole through which Mr Mugabe, a shrewd political operator, can escape from any attempt to undermine his powers.

The key issue is control of the apparatus of repression. Mr Tsvangirai will, apparently, have power over the police, the very men who have beaten him in custody, killed his supporters and tortured those who have dared to challenge the President. That is a fundamental change. So too is the proposed disbanding of the feared Central Intelligence Organisation, a body that has routinely spied on and hounded anyone deemed suspicious by the paranoid leadership, and its replacement with a smaller, more efficient national security authority.

The test will be the behaviour of both bodies from now on. If intimidation of the Movement for Democratic Change stops, the Opposition has indeed won a crucial point. How crucial will depend, however, on whether Mr Mugabe is able to employ troops, who remain under his control, to continue their marauding sweeps across villages loyal to the MDC.

To outsiders it nevertheless seems bizarre and disappointing that a man who has so comprehensively ruined his country should remain at the helm, even if only nominally. Justice demands that he answer for his crimes in a court - in Zimbabwe or at The Hague. But as European Union spokesmen admitted, outsiders are not able to judge how best to alleviate the suffering of ordinary Zimbabweans. With inflation at an all-time record of 11 million per cent, the pressing need is to halt the downward spiral, create jobs and ward off the starvation that stares most families in the face. Dismantling the dictatorship, ending cronyism and dropping radical nationalist policies such as seizing control of banks, mines and other foreign-owned businesses would be a good start. But without massive and immediate foreign aid, there is little chance that Zimbabwe can rise above the despair into which it has sunk.

The world must acknowledge that this is a momentous step - and must perhaps give some credit to Thabo Mbeki, whose diplomacy has at last yielded a result. But Europe, the US and aid organisations will be waiting to see how the deal works. There is a danger that Mr Tsvangirai, a Lech Walesa figure, may find it hard to accept advice or display the agility to outmanoeuvre his enemies. The world's goodwill and the hopes of millions of exiles are riding on him. He will need all the help that he can muster to liberate Zimbabwe from its long nightmare.