Who is to blame for the largest humanitarian catastrophe of our times? I am referring not to Darfur, a terrible tragedy in its own right, but to the situation in the Democratic Republic of Congo, where 4 million people have died in a conflict that has been going on for eight years.

So who is to blame? A U.N. panel sent to Congo to look into illegal arms trading believes it has part of the answer. In a recent report, it explains that Rwanda gave "both direct and indirect support" to dissident officers who mutinied in the eastern Congolese city of Bukavu at the end of May. Many civilians were killed as the troubled country was pushed to the brink of war again.

But while the report accurately describes and condemns Rwanda's meddling, it also constitutes a condemnation of the United Nations itself. As Bukavu was ransacked and hundreds of people were killed, raped or brutalized, the 600 U.N. troops there did little to intervene.

I worked in Bukavu for three years with MONUC, the U.N. mission in Congo. The recent fighting erupted as I was about to leave the region. For me and many of my colleagues, the fighting came as no surprise. It had become obvious that the various factions in the recently formed transitional government were more interested in lining their pockets than in rebuilding their country. Legislation on key issues such as the national army, amnesty and nationality was simply shelved for months as parliamentarians bickered over their salaries and per diems.

What the U.N. panel of experts reported had also been clear to us: that Rwanda was arming and supporting a group of dissenting officers in Bukavu. On several occasions we found arms caches in houses in Bukavu; once we surprised dissident soldiers secretly transporting weapons stolen from the national army.

Despite this knowledge, MONUC did little to prevent the violence. The Security Council imposed a ban on military and financial support of armed groups in eastern Congo last July. But the only concrete step taken was to create a panel of experts that finally visited Bukavu for the first time a month ago. No U.N. soldiers were placed at the borders and no boats patrolled Lake Kivu, which separates Rwanda from Congo. And when reports streamed in that Rwanda -- Congo's pint-size neighbor, which has sponsored two previous Congolese rebellions -- was arming the dissidents in Bukavu, MONUC did nothing.

On May 26 the lakeside city of 600,000 was caught up in all-out urban warfare. Few soldiers died in the battle, but, as always in Congo, the belligerents seized the opportunity to loot and rape. As we walked into neighborhoods at night on patrol, dozens of voices from all around screamed for help as soldiers broke down doors and smashed windows. Soldiers dragged a Tutsi civilian, a member of the same ethnic group as the dissidents, out of the hospital's intensive care ward and beat him to death with bricks.

MONUC's reaction was fitful and ill-planned. After three days of fighting, we managed to canton the dissident troops in town, only to let them escape in the middle of the night. As more dissidents arrived from the airport, we received orders from our headquarters not to intervene in what it considered internal affairs of the Congolese. And as the town was looted from top to bottom, U.N. spokesman Fred Eckhard said: "It's for the [Congolese] parties to sort out. When war breaks out, the role of peacekeepers ends."

The insurgency is far from over; the dissidents are regrouping in Goma, to the north of Bukavu, and Congolese President Joseph Kabila has sent some 10,000 soldiers to the east. The scene seems to be set for a showdown.

MONUC's mandate has been renewed for two months as the Security Council considers fundamentally changing the mission. Secretary General Kofi Annan has pressed for a more robust U.N. force, with more troops and a greater degree of leverage on the Congolese government. It is up to the Security Council and especially the United States, which foots the bill for about a third of MONUC's budget, to strengthen the faltering U.N. mission. While Sierra Leone, a country one-twentieth the size of Congo, currently has 15,000 U.N. soldiers, Congo has a meager 10,000. In addition, as the recent events in Bukavu have shown, MONUC needs to scrutinize the way it operates and how it is organized to prevent further blunders.

These changes will be difficult. Congo is competing not only with Iraq and Sudan for the attention of the international community but also with other U.N. missions in Liberia, Haiti and Burundi that are just taking off. But if the growing crisis in eastern Congo is not taken seriously, Bukavu will be just the first of numerous nails in the coffin of the nascent Congolese state, and another stage in an immense human catastrophe.

The writer recently returned from the Democratic Republic of Congo, where he worked for three years, first as a human rights advocate and then as a demobilization officer for the U.N. peacekeeping mission.