A FEW MONTHS AGO, as he addressed U.S. and other peacekeeping troops in Macedonia, President Clinton made some remarks that he has now had occasion to extend and revise. He talked then about how proud he was of Wes Clark, the general who led NATO's Kosovo campaign; shortly thereafter, the president decided to relieve Gen. Clark of his position ahead of schedule. More substantively, Mr. Clinton delivered, in the wake of Serbia's defeat in Kosovo, an expansive view of America's ability and determination to intervene in humanitarian missions: "Whether you live in Africa or Central Europe, or any other place, if somebody comes after innocent civilians and tries to kill them en masse because of their race, their ethnic background or their religion, and it's within our power to stop it, we will stop it."

This week, in a speech to the United Nations General Assembly, Mr. Clinton scaled back that promise. "We cannot do everything everywhere," he said. The president, who on a trip to Africa dramatically apologized for his failure not to do more to prevent the genocide in Rwanda, now says, "We must approach this challenge with some considerable degree of humility. It is easy to say, never again, but much harder to make it so. Promising too much can be as cruel as caring too little."

Mr. Clinton still hasn't come up with an unassailable doctrine, but his acknowledgment of the difficulties and complications is more attractive than his earlier glib assurances. These questions, as U.N. Secretary General Kofi Annan said in a speech to the same body on Monday, "do not lend themselves to easy interpretations or simple conclusions." Mr. Annan wasn't happy when NATO bombed Serbia without specific U.N. authorization; but neither could it be right, he admitted Monday, to sit back and allow "gross and systematic violations of human rights." Crimes against humanity are as wrong in Rwanda as in Sudan or Kosovo or East Timor. Yet it's also true that the United States can't lead every fight. How to balance humanitarian impulse with national interest, how to persuade other nations to play a part -- these need to be openly debated.

The East Timor peacekeeping force arrived too late, but its deployment with U.N. blessing is a good sign; so is the precedent that the United States need not be in the forefront each and every time. It's also true that much of the answer lies, as Mr. Annan said, in preventive diplomacy; NATO's stark choice in Kosovo between force and abdication in itself resulted from earlier failures of resolve. But that NATO was right to act, even without U.N. blessing, cannot be doubted. It's encouraging that the United Nations is now reflecting on its inability to rise to that challenge.