SOUTH AFRICA has won a round in its relentless campaign to preserve Robert Mugabe’s hold over a dying Zimbabwe. With the help of its allies in the Southern Africa Development Community, South Africa succeeded last week in coercing opposition leader Morgan Tsvangirai — the winner of last year’s presidential election — into accepting a subordinate role in a “unity” government led by the 84-year-old strongman. The deal, which Mr. Tsvangirai bravely resisted for months, will leave Mr. Mugabe in charge of the country’s last functioning institutions — army and police forces that have been waging a campaign of murder, rape and torture against the opposition and human rights activists.

Mr. Tsvangirai relented because he believed that the frightful humanitarian emergency in Zimbabwe left him with little choice. The United Nations estimates that 7 million of the 9 million people remaining in the country need food aid this month. A cholera epidemic has so far infected more than 62,000 and killed 3,100. Schools, hospitals and most businesses have closed, the national currency has been discarded and unemployment is over 90 percent.

The opposition will be placed in charge of the finance, health and education ministries, which it hopes will allow it to solicit and distribute aid to prevent mass death from starvation and disease. As South Africa and its client more cynically calculate, Mr. Tsvangirai’s appointment will compel the United States, Britain and other Western governments to lift sanctions and renew economic support, thus preventing what would otherwise be the inevitable collapse of Mr. Mugabe’s regime.

The misery of Zimbabwe is indeed compelling — but the Obama administration and other Western governments should reject South Africa’s demands. It long ago became clear that Zimbabwe cannot recover as long as Mr. Mugabe remains in power. South Africa and other neighbors who insist on supporting the criminal regime are free to supply aid. But Western governments must maintain their sanctions — especially those aimed at individual members of the Mugabe regime and the companies they control.

A State Department statement this week said the administration would consider new assistance and the lifting of sanctions “when we have seen evidence of true power sharing as well as inclusive and effective governance.” What should that include? Mr. Tsvangirai himself is demanding the freeing of more than 30 opposition activists from prison. Legislation must be passed giving the opposition a measure of control over security forces, and replacing the central bank president — a Mugabe crony — with a technocrat. Restrictions on the press must be lifted and foreign journalists admitted. Perhaps most important, the government must agree on a plan for a new presidential election, with guarantees for fairness and full international monitoring.

If these steps were taken, Western aid to Zimbabwe might serve some purpose. But they won’t be. “Zimbabwe is mine” is Mr. Mugabe’s only principle. The first step in any rescue must be prying the country from his grip.

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