While it has dominated headlines in Sudan, the expulsion of Jan Pronk, the United Nations' special envoy there, is really just a sideshow. He would have been leaving his post at the end of the year in any case. The great moral and geopolitical challenge for the UN and its member states is to muster the political will to stop the genocide in Darfur.  The expulsion reveals the crude cynicism of the National Islamic Front regime in Sudan, a gang that has been murdering, raping and uprooting hundreds of thousands of villagers in Darfur. Pronk, in fact, has favored the regime's proposal to beef up the current ineffective African Union force of 7,500 monitors in Darfur rather than implement a Security Council resolution calling for a UN peacekeeping force of 22,000.  In his private blog, the former Dutch Cabinet minister wrote that because of renewed warfare in northern Darfur, Sudanese soldiers are deserting, generals are being cashiered and troop morale is plummeting. Sudan's military brass accused Pronk of waging "psychological war against the Sudanese army" and declared him persona non grata.  Pronk had been warned in the past by UN headquarters against undiplomatic blog postings; he had to know the genocidal regime in Khartoum would react as it did.  Whether intended or not, the one clear benefit of Pronk's indiscretion is to focus attention on the looming danger for 2 million people in Darfur and neighboring Chad whose survival is at risk if the United Nations fails to undertake the lifesaving mission it set for itself in Security Council resolution 1706.  That resolution authorizes the rapid deployment of 22,500 military and security personnel to Darfur to protect civilians at risk as well as humanitarian operations, and also to extend security to the increasingly violent border area between Darfur and Chad.  Sudan is refusing to accept the UN peacekeepers. The recent intensification of violence evoked by Pronk means that hundreds of thousands of defenseless tribal people in Darfur could lose their lives in coming months. So there is no time to pretend the Khartoum regime can be cajoled into some new peace agreement with rebel factions in Darfur. And narrow sanctions on a few notables in that regime can hardly end the genocide.  There must now be a willingness to send a robust UN force to Sudan - even without the consent of the regime. To avoid an unprecedented confrontation, UN Secretary General Kofi Annan should ask the Arab League to prevail on Sudan to accept the UN mission. But if the League refuses or Sudan refuses the League's entreaties, the Security Council should act to save the lives of Darfur's 2 million internal refugees. The choice is stark: to end a genocide, or let it continue.