FOR YEARS, a U.N. peacekeeping force in Congo was a glaring testament to the organization’s fecklessness. The largest such force in the world, with 19,000 troops, the “stabilization mission” cost $1.5 billion a year but utterly failed to pacify eastern Congo, which has been a battleground for warlords and the armies of neighboring powers for nearly two decades. At its low point a year ago, the U.N. blue helmets watched passively as a vicious force of defectors from the Congolese army occupied and ravaged the city of Goma.

Now, at last, the U.N. Congo mission has a victory to savor — as well as a potential model for future peacekeeping missions. This month a special U.N. “intervention brigade,” made up of 3,000 soldiers from Tanzania, South Africa and Malawi, joined the Congolese army in routing the defectors, whose M23 militia had been wreaking havoc for 18 months. Fleeing to Uganda, which, along with Rwanda, had been their sponsor, the rebels disbanded and were disarmed. Their military commander, Sultani Makenga, who is wanted for war crimes, is being held by the Ugandan army.

A peace deal between Congo’s government and the rebels fell through last week, prompting Uganda to warn that the M23 could revive. But thanks to the United Nations’ unprecedented use of muscle, the region has its best shot at peace in years. The force, authorized last March by the U.N. Security Council, is the first with a mandate to undertake offensive operations. Its presence helped discipline the Congolese army and — combined with heavy pressure from the United States and European Union — induced Rwanda to cease its support for the rebels.

Encouragingly, the capable Brazilian commander of the U.N. force, Gen. Carlos Alberto dos Santos Cruz, has been saying he intends to follow up by taking on the score of smaller militias still operating in eastern Congo. The most significant is the Democratic Forces for the Liberation of Rwanda, which was formed by ethnic Hutu extremists who carried out the 1994 genocide against Rwandan Tutsis. Until that force is eliminated, Rwanda will retain an incentive — an excuse, some say — for backing Congolese proxies.

In the meantime, the success in Congo ought to prompt some reflection in the United Nations’ department of peacekeeping operations, which oversees nearly 100,000 troops all over the world. Many of its operations have been embarrassing failures: Consider the mission in southern Lebanon, which since 2006 has stood by while the Hezbollah militia has deployed tens of thousands of missiles aimed at Israel, despite a Security Council mandate to “ensure that its area of operations is not utilized for hostile activities of any kind.” More forces prepared to take the offensive could prevent such debacles or even reverse them; that is what has happened in Congo.