Sudan's National Islamic Front regime has begun its sixth year of genocidal counterinsurgency warfare in the western region of Darfur, targeting African civilian populations perceived as the primary support for fractious rebel groups. Given the length of the conflict, news reports have inevitably taken on a grimly familiar and repetitive character that obscures the impending cataclysm of human destruction.

Without significant improvement in security — for civilians and the humanitarians upon whom they increasingly depend — deaths will reach a staggering total. What Khartoum was unable to accomplish with the massive violence of 2003-04, entailing wholesale destruction of African villages, will be achieved through a "genocide by attrition." Civilians displaced into camps or surviving precariously in rural areas will face unprecedented shortfalls in humanitarian assistance, primarily food and potable water.

A recent U.N. map indicating aid access throughout Darfur shows that a large majority of people in the region are in areas with highly limited humanitarian access or none at all. The consensus among nongovernmental aid organizations is that they have access to only 40 percent of the population in need; 2.5 million of the 4.3 million Darfuris affected by conflict — primarily women and children — can't be securely reached by those attempting to provide food, water, shelter and medical care.

Things are poised to get worse.

Paralyzing seasonal rains begin in earnest in June. In eastern Chad, an obscenely underreported humanitarian crisis has put half a million Darfuri refugees and Chadian displaced persons at acute risk because of insecurity spilling over from Darfur. A European Union force deploying to eastern Chad may provide some of the protection necessary to halt the most threatening violence, but much depends on whether the force is perceived as an extension of a long-term French military presence that has supported Chadian President Idriss Deby.

In Darfur itself, however, the protection force authorized by the U.N. Security Council last July has stalled badly. Little more than a slightly augmented version of the African Union mission, it risks failing soon if it cannot do much better than its weak and undermanned predecessor. Khartoum refuses to accept key contingents from non-African countries and obstructs force deployment and operations in a range of ways.

Nothing contributes more to what Human Rights Watch recently described as "chaos by design." While a variety of rebel groups and opportunistic armed elements contribute to the violence that threatens humanitarians, Khartoum has invested virtually nothing in providing security for Darfuris or humanitarians. Reports from the field make clear that a climate of hostility, obstruction and abuse defines the working environment for all aid organizations. Khartoum refuses to disarm its brutal Arab militia forces, the Janjaweed. Recently, in a campaign reminiscent of the worst military violence of the genocide's early years, Khartoum's regular ground and air forces coordinated with the Janjaweed in massive scorched-earth assaults against villages.

But it is the onset of this year's heavy rains that may well mark the tipping point. Many people weakened by five years of conflict and deprivation won't make it through the traditional "hunger gap" — the period between spring planting and fall harvest. Last fall's harvests were disastrous. Food reserves have never been lower, and because of insecurity the U.N. World Food Program has not been able to position adequate food stocks in the areas least accessible during the rainy season. Once the rains come it will be almost impossible to move in many areas.

The international community has waited far too long to come to terms with the brutal motives behind Khartoum's simultaneous blocking of a U.N.-authorized protection force and its unconstrained harassment of humanitarian operations. Nothing short of the most urgent deployment of security forces will allow food to be moved into areas of greatest need. And nothing less than an equally urgent commitment to protect aid operations will permit an expanded humanitarian reach in the critical three months before the start of the rainy season. If Khartoum is not confronted over its deadly policies of fostering insecurity while obstructing humanitarian operations, then we may measure the consequences in hundreds of thousands of lives lost. The choice is before us now.

Eric Reeves is a professor at Smith College and a consultant to several human rights and humanitarian organizations in Sudan. He wrote this commentary for The Washington Post.