UNITED NATIONS — Three years after the United Nations adopted a groundbreaking resolution to help it intervene to stop genocide, even longtime supporters of the rule acknowledge that it has not helped the organization end the violence in Darfur.

The General Assembly resolution, approved in 2005, held nations responsible for shielding their citizens from mass atrocities and established the right of international forces to step in if nations did not fulfill this new “responsibility to protect.”

“It was the high-water mark when the General Assembly endorsed the concept; it was an incredible leap forward from the whole crippling debate over whether humanitarian intervention wasn’t just a Trojan horse for neo-imperialism,” said John Prendergast, co-chairman of the Enough Project, a Washington-based group dedicated to preventing genocide.

“When it happened in 2005,” he said, “you believed that potentially things could be different. But in the daily slugfest of international policy making, it hasn’t survived the first test: Darfur.”

The United Nations has tried to take the lead in Darfur, the crisis-ridden region in western Sudan. But it has been stymied by the failure of major member states to fulfill promises to support action and by the intransigence of the Sudanese government.

Sudan begrudgingly agreed last year to permit United Nations peacekeepers into Darfur but only as part of a joint mission with the African Union, whose own 7,000-member force had proved inadequate.

Since then, the government has thrown up so many bureaucratic and operational roadblocks that the force that took over on Jan. 1 is only a third of its planned strength of 26,000, and Sudanese authorities are still blocking United Nations’ efforts to include specialized non-African troops considered essential to making the mission effective.

In addition, countries with advanced militaries have not come forward to answer United Nations appeals for the sophisticated aviation and logistics assistance that the force needs.

Darfur, in short, has shown that there is a great difference between gaining acceptance for a working theory and making the theory work.

The 2005 resolution was meant to break the impasse between those who believe the outside world has the power to intercede in countries where mass atrocities are occurring and those who believe that the sovereignty of the state, a concept created in the 17th century and recognized in the United Nations Charter, precludes any outside intervention.

The phrasing of the resolution sought to square the two long-antagonistic positions by saying that the world could step in, but only after the state had shown unwillingness to act itself.

The longstanding debate over when countries should intervene took on urgency after the United Nations and its principal member states on the Security Council did nothing to stop the 1994 genocide in Rwanda, where Hutu extremists slaughtered 800,000 Tutsi and moderate Hutu. Despite desperate calls for reinforcements from the United Nations commander, Gen. Roméo Dallaire of Canada, the Security Council cut the number of peacekeepers to 450 from 2,500.

Donald Steinberg, the New York director of the International Crisis Group, remembers his despair that year when, as President Clinton’s special assistant for Africa, he was unable to marshal international support for taking action to stop the killing.

Among the available options, he recalled, were jamming the radio station broadcasting tribal hate messages, reinforcing United Nations peacekeeping forces or immediately declaring the situation to be genocide.

“But each time some of us pushed for these steps,” he said, “others would ask, ‘Where’s the legal basis for these actions, where’s the public outcry, the ‘Hallelujah’ chorus of support? Where’s the evidence to show that these actions will end the killings?’ ”

With the world facing in Darfur a situation that many have identified as genocide, the advocates of international intervention should, in theory, have answered those questions.

First, there is Security Council approval for the largest peacekeeping force in history, which, at full strength, should have the capacity to halt the killing.

Second, there is a vocal, organized and worldwide campaign backing intervention.

As for the legal basis, there is the 2005 General Assembly resolution embodying the concept of a state’s “responsibility to protect,” which has become so much a part of the United Nations vocabulary of resolving conflict that it even has its own abbreviation, R2P.

Secretary General Ban Ki-moon has put so much faith in it that he uncharacteristically upbraided an under secretary general at a high-level policy committee meeting in October who disputed the high priority being placed on the concept of intervention.

“The S.G. said he utterly disagreed and felt that this was fundamental to the future of the U.N. and that after elevating the principle so high, we had the obligation to put it into effect,” said a participant in the meeting who witnessed the exchange and agreed to talk about it in exchange for anonymity.

Mr. Ban has upgraded and broadened the post of special adviser for the prevention of genocide and mass atrocities and created a new assistant secretary general position specifically on the responsibility to protect.

But many of the developing-world countries that supported the resolution three years ago have backed off out of suspicion that they could become targets of intervention.

“There has been a tremendous amount of buyer’s remorse,” Mr. Steinberg said.

Samantha Power, a professor at the Kennedy School of Government at Harvard, said, “We have more than 150 countries on the books saying they believe this responsibility exists, but what advocates have begun to understand is that governments will never exercise this responsibility naturally or eagerly, they will only exercise it if they feel they are going to pay a price for not exercising it.”

At the same time, advocates of intervention say the resolution has already shown at least some value.

“I think it’s the best tool we’ve come up with for educating; it just remains to be seen if it will be as good at converting theory to action,” said Mr. Prendergast, of the Enough Project.

Mr. Steinberg said, “It’s a way of telling people that sovereignty is not an excuse to facilitate mass killings in your own country, and people get that.”

Ruth W. Messinger, president of the American Jewish World Service and a co-founder of the Save Darfur Coalition, said, “I think it’s a critically important phrase, and I don’t say that lightly.”

Next month, a research and advocacy center dedicated to moving the principle of responsibility to protect into practice is being inaugurated at the Ralph Bunche Institute for International Studies at the Graduate Center of the City University of New York. Similar offices are being set up in Australia, Sri Lanka and Thailand.

Recalling passage of the declaration in 2005, Mr. Steinberg said, “We decided then that this was the most the market would bear, but we haven’t gotten what we need out of it, and unless we can apply it to a situation like Darfur, then the promise will be lost.”