The first gauntlet thrown at President Obama didn’t come from Iran, Russia or China. Rather, it came from Sudan, in its decision to expel aid groups that are a lifeline keeping more than a million people alive in Darfur.

Unfortunately, the administration’s initial reaction made Neville Chamberlain seem forceful. The State Department blushingly suggested that the expulsion “is certainly not helpful to the people who need aid.”

Wow.

Since then, the administration has stiffened its spine somewhat. Susan Rice, the ambassador to the United Nations and designated hitter on Sudan, told me, “If this decision stands, it may well amount to genocide by other means.”

That’s exactly what we may be facing, for President Omar Hassan al-Bashir is confirming the International Criminal Court’s judgment when it issued an arrest warrant for him on Wednesday for “extermination,” murder and rape. Now Mr. Bashir is preparing to kill people en masse, not with machetes but by withholding the aid that keeps them alive.

More than one million people depend directly on the expelled aid groups for health care, food and water. I’ve been in these camps, so let me offer an educated guess about what will unfold if this expulsion stands.

The biggest immediate threat isn’t starvation, because that takes time. Rather, the first crises will be disease and water shortages, particularly in West Darfur.

The camps will quickly run out of clean water, because generator-operated pumps bring the water to the surface from wells and boreholes. Fuel supplies to operate the pumps may last a couple of weeks, and then the water disappears.

Health clinics have already closed, and diarrhea is spreading in Zam Zam camp and meningitis in Kalma camp. These are huge camps — Kalma has perhaps 90,000 people — and diseases can spread rapidly. Children will be the first to die.

Hundreds of thousands of people in the camps may try to flee to Chad, but that would overwhelm Chad’s own impoverished and vulnerable population. And to top it off, Mr. Bashir has armed a large proxy force of Chadian rebels who are said to be preparing an attack on the Chadian government.

“This is a whole new kind of hell for the people of Darfur,” Josette Sheeran, the head of the United Nations World Food Program, told me. “The life bridge for more than a million people has just been dismantled.”

My hunch is that Mr. Bashir’s calculation is twofold. First, he hopes that if there’s enough suffering in Darfur, the United Nations Security Council will approve a one-year delay in the court’s proceedings (he miscalculated, for that won’t happen). Second, he has long wanted to get rid of aid workers in Darfur, partly because they are the world’s eyes and ears there.

I was on the Chad-Darfur border a couple of weeks ago, talking to Darfuri refugees, and they worried that Mr. Bashir might lash out after an arrest warrant. But they still rejoiced at the prospect, as a sign that the deaths of their loved ones mattered and as a sign that impunity for murder and rape might be coming to an end. Not a single Darfuri I spoke to favored a delay in International Criminal Court proceedings.

Our greatest problem in responding to Darfur is that we have never held either carrots or sticks. It’s difficult at this point to offer carrots, but the United States and other countries can wield some sticks.

Gen. Merrill McPeak, the former Air Force chief of staff and a co-chairman of the Obama presidential campaign, suggested one in an op-ed article in The Washington Post on Thursday: a no-fly zone over Darfur. The aim is to attach costs to brutality and gain leverage.

Sudan cares deeply about maintaining its air force, partly because it is preparing for renewed war against South Sudan. That means that a denial of air cover or the loss of helicopter gunships would deeply alarm Sudan’s military, and that gives us leverage.

Another option is for the government of South Sudan to take over administration of Darfur. The leaders of South Sudan have periodically offered to send 10,000 of their troops into Darfur, and if the north Sudanese government cannot provide security or look after Darfur’s needs then the south can try, with international backing.

Madeleine Albright, the former secretary of state, says she was intrigued by General McPeak’s proposal for a no-fly zone and adds, “I don’t think the international community can stand by and watch as thousands more people starve to death.”

“We were criticized, rightfully so, on Rwanda,” Ms. Albright said. But she noted that the Rwandan genocide ended quickly, while Darfur has dragged on for years. “You can’t watch this and not feel that there has to be something done,” she said.