KIWANJA, Democratic Republic of Congo -- When Martin Kobler, the newly appointed United Nations representative, arrived this summer for his first visit to this small but strategic town, armed members of the M23 rebel group lined the airstrip, silently watching him land and disembark. Their presence sent an unmistakable message: the rebels, not Congolese officials, controlled Kiwanja.
But when he landed at the end of October, he was greeted instead by crowds of cheering civilians. The armed fighters who had terrorized them were nowhere in sight. Congolese forces, supported by United Nations peacekeepers, had routed the rebels and restored control of the town to the central government.
''Our task is to dissolve political blockage, to end occupation by armed forces, to restore state authority, to bring back hope to the people,'' said Mr. Kobler, a German career diplomat and special representative of the United Nations secretary general, Ban Ki-moon. ''You have to find new instruments to restore the peace.''
The new instrument in question was the Force Intervention Brigade, made up of 3,000 soldiers from South Africa, Tanzania and Malawi. Rather than waiting for attacks, the United Nations Security Council authorized the troops to ''neutralize armed groups.'' It was a major departure from the often passive approach that has given peacekeepers a bad reputation, from failing to prevent the slaughter of Bosnian Muslims in Srebrenica to not intervening to stop the massacres of Tutsis and moderate Hutus in Rwanda.
Those instructions, delivered with rare agreement among members of the Council, were certainly a gamble. They aimed not only to restore hope to the long-suffering people of Congo, but also to rehabilitate the image of the United Nations peacekeepers, under whose watch a massacre took place in this very town in 2008. For the moment, the gamble appears to have paid off, even as it raises new risks.
The pitfalls of backing one side in such a conflict were on full display this week after the Congolese government walked out on peace talks with the rebels.
''Despite a change in the military situation, it is important that there be a political conclusion to the dialogue,'' said a group of envoys representing the international community in a statement on Monday, including Mr. Kobler and his counterparts from the United States, African Union and European Union.
Still, the intervention brigade, combined with forceful diplomatic pressure and financial incentives for neighboring Rwanda, has made a dent in a seemingly endless war.
''I think it has contributed to rebuilding the credibility of the U.N., which was almost nonexistent in the Congo after years of humiliation,'' said Jean-Marie Guéhenno, who was the United Nations peacekeeping chief from 2000 to 2008, and under whose watch its blue helmets were overwhelmed by rebel forces in eastern Congo.
The shift could have broad implications for peacekeeping operations all over the world. Nearly 100,000 uniformed personnel serve under the United Nations Department of Peacekeeping Operations, from the Western Sahara and Haiti to the island of Cyprus and the mountains of Kashmir.
The United States special envoy to the Great Lakes region of Africa, Russ Feingold, described the intervention brigade as ''a stronger approach that can give peacekeeping operations more strength in the future and help resolve knotty problems.''
He added, ''The story has yet to be written, but the first couple chapters are very good.''
Samantha Power, the United States ambassador to the United Nations, described the new brigade as having ''invigorated'' the efforts of both Congolese troops and the rest of the United Nations mission.
The decision to send the blue helmets into offensive operations has not met with universal approval. Humanitarian aid organizations worry that the shift could put their workers at risk because armed groups will not distinguish between soldiers and those who feed, heal and house civilians in war.
''You can have a helicopter one day used to deliver the Force Intervention Brigade troops to attack a village and next day to deliver aid to that same village,'' said Michiel Hofman, senior humanitarian specialist for Doctors Without Borders in Brussels. ''In this case it's not even a blurring of the lines.''
Mr. Guéhenno, who now teaches at Columbia University, warned against relying too much on force, saying that the key to ending the war in Congo, is persuading regional leaders to cooperate, out of their own mutual self-interest.
''It's not a SWAT team that's going to clean up a bad neighborhood,'' Mr. Guéhenno said. ''That requires politics.''
A more aggressive approach could also alienate countries like India and Uruguay, which have traditionally sent many troops to serve in United Nations operations. Such countries see peacekeeping missions as a way to get training, equipment and extra pay for their forces with relatively little risk of casualties.
''Some of the troop-contributing countries are quite uneasy with what they see as the direction peacekeeping is taking,'' said a senior United Nations official, speaking on the condition of anonymity because he was not authorized to talk publicly.
The Security Council was ''careful to say it was not a precedent, but every time you say that that's exactly what you're making,'' the official said.
Mr. Kobler's military counterpart in Congo, whom he describes as his ''twin,'' Lt. Gen. Carlos Alberto dos Santos Cruz, has used all the soldiers under his command more aggressively and said that he has no plan to change tactics.
''We are going to exercise our mandate to the maximum possible, not only against M23, against all the groups,'' General Cruz said. ''When we finish one problem, we are in our heads thinking about the next step.''
Dozens of armed groups remain in eastern Congo, a region where millions have died in a war that has drawn in the country's neighbors and raged for two decades. The Congolese Army is also part of the problem: It has been cited for wide-ranging human rights abuses for years. Analysts warn that the defeat of a single group like the M23, which was plagued by internal dissent and weakened even before the latest offensive, is no reason for triumphalism.
Intense diplomatic pressure on Rwanda to cooperate and financial incentives, namely a handsome package from the World Bank, also helped.
The United Nations force in Congo includes nearly 19,000 military personnel and costs nearly $1.5 billion a year.
Mr. Kobler, an experienced German diplomat, served stints as ambassador in Egypt and Iraq, and more recently for the United Nations in Afghanistan.
Wiry and energetic, Mr. Kobler has ricocheted across the region to seek an end to the fighting. The day the most recent round of fighting erupted last month, Mr. Kobler flew immediately from the Congolese capital, Kinshasa, to the eastern city of Goma, a thousand miles away, to tour the front line. The next day he flew to Kigali, Rwanda, for meetings, then on to Kampala, Uganda, before ending the weekend here.
He returned the following day with the governor of the province. On the helicopter ride there he peered out the window at the farmed hillsides, waving to farmers and children below as if he could conduct public relations for the United Nations from the air.
The visit included a memorial service for a fallen Tanzanian peacekeeper, Lt. Rajabu Ahmed Mlima, 36, who was killed by rebel fire under a blooming bougainvillea as the intervention brigade fought the rebels. His death was a reminder that aggressive military tactics can exact a heavy toll. To the low thrum of helicopter rotors overhead, Mr. Kobler and other dignitaries laid garlands over a framed photograph of Lieutenant Mlima.
''This is a sad moment. We see one of us depart with the ultimate sacrifice,'' Mr. Kobler said. ''He lost his life for the values of the United Nations, for protecting civilians, for defending human rights, for fighting for the benefit of the Democratic Republic of Congo.''