The man accused of attempting to exterminate whole tribes in Darfur flew there yesterday for the first time in a year. His alleged victims are buried in shallow graves across large swaths of Sudan. Survivors of the raids and bombing runs he is said to have authorised huddle in vast refugee camps that he has never visited.

According to the United Nations the ethnic cleansing unleashed by President al-Bashir has left 300,000 dead and 2.5 million homeless. It has used rape as an instrument of terror and fuelled a two-year war with Chad. Mr Bashir, whose stock in trade is brazen denial, arrived in El Fasher to proclaim himself a man of peace and smile indulgently as three fat doves were released in his honour.

Mr Bashir is not a man of peace. There is a small mountain of evidence to suggest he has personally orchestrated the horror visited on Darfur since 2003, and last week the chief prosecutor of the International Criminal Court (ICC), Luis Moreno-Ocampo cited Mr al-Bashir under the Court's remit of prosecuting crimes of genocide, crimes against humanity, and war crimes.

The evidence Mr Moreno-Ocampo is now studying is compelling. Justice, the long-term stability of the region and the reputation of the ICC all demand that he answer any charges against him in due course, as Radovan Karadzic will do shortly at the International Criminal Tribunal for the former Yugoslavia. But more immediate considerations require the ICC to proceed carefully.

When Mr Moreno-Ocampo's citation was announced, the streets of Khartoum filled with pro-Government protesters denouncing both the court and aid workers whom they accused of fabricating evidence against their leader. The aid community in Sudan, a lifeline for millions, feared a violent backlash. It has not come, but that danger will return if a full indictment is issued while Mr Bashir remains in office. Darfur's refugees themselves are similarly at risk: a full-scale resumption of ethnic cleansing is entirely possible should Mr Bashir decide his current diplomatic offensive is pointless.

The third danger that attaches to any precipitate action by the ICC concerns legal precedent. Hitherto, serving heads of state, including Robert Mugabe on his recent visit to the UN food summit in Rome, have enjoyed immunity from prosecution under international law. To indict Mr Bashir while still in office would break new legal ground. It would put all serving heads of state at risk of being grounded by indictments, whether serious or frivolous, and it would put the court's proceedings at grave risk of being skewed by politics.

What if a zealous prosecutor were to conclude that the British decision to participate in the Iraq War was a war crime? It would be conceivable, but a gross example of a judicial institution substituting its own judgment for the affairs of state. The same is simply not true in the case of Mr Bashir. Sovereignty is not licence. No ruler has a right to inflict such a catastrophe. Where any does, the international community must stop him. Sometimes the best possible outcome is the enemy of the attainable improvement.

The case against Mr al-Bashir should be assembled rigorously. But his prosecution should await his departure from office.