For a man who has battled for nearly a decade to become President of Zimbabwe, Morgan Tsvangirai's decision to pull out of the race against Robert Mugabe only days before polling must have been the toughest of his career.

The former union boss has suffered arrest and beatings at the hands of his rival. His supporters have been murdered, arrested and tortured. Many wanted him to continue the fight until election day on Friday, but he reached the conclusion that staying in the presidential race would only lead to more bloodshed.

As Mr Mugabe has made clear in both words and deeds over the past week, the outcome of the vote was never in doubt. A clear majority of Zimbabweans may want him out, but there is not much that the civilian population can do against the combined might of the Zanu (PF) militia and the security forces, not to mention the electoral authorities, which have still not given a full account of the last vote. In Mr Mugabe's own words, “only God” could remove him from office.

Mr Tsvangirai has probably rightly calculated that he can still prevail, but that he will have to adopt different tactics to oust Mr Mugabe. The battle will now move beyond the country's borders to the international community, in particular the 14-nation Southern African Development Community (SADC), the African Union and the United Nations.

In the past South Africa, and in particular President Mbeki, has shielded Mr Mugabe from outside pressure. There is now growing evidence that Zimbabwe has become an embarrassment for African leaders and a burden on its neighbours, where millions of Zimbabweans have sought sanctuary.

SADC members such as Zambia, Tanzania, Angola and Botswana have broken ranks to criticise Mr Mugabe's regime. He has also come under regular attack from South Africa's ruling African National Congress and its leader, Jacob Zuma, Mr Mbeki's likely successor next year.

The aim of the anti-Mugabe forces will be to turn Zimbabwe into a pariah state. As a first move, the international community can refuse to recognise Mr Mugabe when he is sworn in for another term as President. Sanctions can be increased against the leadership, particularly any figures responsible for the latest violence. If South Africa throws its full weight behind the effort, the regime in Zimbabwe will be doomed.

Britain and America are likely to press for action from the United Nations Security Council, where even Zimbabwe's old allies, such as China, may distance themselves from his regime.

While political pressure grows from abroad, the unopposed re-election of Mr Mugabe is likely to lead to the final collapse of the Zimbabwean economy. The regime may still retain the loyalty of party activists and the security forces, but its ability to function will be seriously tested amid spiralling inflation and growing food shortages.

The combined political and economic forces squeezing the country should make it clear to the regime that it is not possible to turn the clock back to the situation that existed before the first-round elections in March, which Mr Tsvangirai and his Movement for Democratic Change won.

It is not now a question of whether Mr Mugabe's regime will collapse, but when. The worst possible outcome would be a descent into further conflict and the need for an international intervention force to restore order.

Hopefully before that happens the veteran Zimbabwean leader can be persuaded to bow out peacefully. That could allow some political settlement to be reached, perhaps a transitional period leading to fresh elections. Mr Tsvangirai may yet succeed Mr Mugabe, without the need for divine intervention.

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