The career of U.N. Secretary-General Kofi Annan, which will come to an end Dec. 31 with the completion of his second five-year term, has been bookended by dramatic peacemaking trips to the Middle East.

During the first, in February 1998, he headed off an impending U.S. bombing attack on Iraq by persuading Saddam Hussein to permit U.N. weapons inspectors to return to the country. The Daniel-in-the-lions'-den imagery of that meeting cemented his reputation as a moral hero.

In August, the 68-year-old Ghanaian returned from a lightning tour of Middle Eastern states with an agreement to lift Israel's suffocating embargo of Lebanon and reassurances about the brittle cease-fire established by a Security Council resolution.

This is the stuff of diplomatic melodrama. The Baghdad entente of 1998 began to fall apart soon after Annan returned to New York, and by December, British and American warplanes were striking targets in Iraq. As for the recent cease-fire between Israel and Hezbollah, it could give way at any moment because the United Nations lacks the mandate and the capability to disarm the Shiite militia.

In both cases, Annan did everything humanly possible, but a secretary-general can do very little to alter the fundamental calculations of states. As the United Nations begins to choose a replacement for Annan, we must bear in mind that a secretary-general is a creature of the geopolitical moment.

From 1997 through 2000, the world was largely at peace, none of the horrific civil wars in the Third World rose to genocidal proportions, and the White House was occupied by an internationalist Democrat.

Then, in rapid succession, a unilaterally minded Republican took office, 9/11 shattered the interval of peace, the U.S. invaded Iraq in the face of international opposition and one of Sudan's interminable ethnic conflicts erupted into a scorched-earth war. The man who had won the Nobel Peace Prize in 2001 looked hapless, even pitiful.

Annan's detractors see it differently. They believe that he is very much the author of his own misfortunes. In late 2004, Sen. Norm Coleman, R-Minn., called for the secretary-general to resign, accusing him of tolerating an atmosphere of corruption inside the U.N. as the Iraqi oil-for-food scandal unfolded. Other critics in the West have insisted that Annan is hostile to Israel and to Washington's legitimate security interests.

In the developing world, Annan is regularly denounced as Washington's puppet. Others with no ideological ax to grind, including U.N. insiders, feel Annan is far too much a creature of the U.N. to change its entrenched culture of patronage and idle paper-pushing.

But secretaries-general should be judged according to what they can do, and not what they can't. Annan did not get the Nobel for temporarily averting war in Iraq. He did something harder: He restored the relevance of the U.N. The new U.N. head brought Washington back into the fold through a combination of unflagging solicitude and gentle prodding. Annan's promulgation of a new spirit of self-criticism brought the U.N. to anatomize the mistakes that led to the peacekeeping fiascos in Somalia, Bosnia and Rwanda in the 1990s. This, in turn, encouraged the U.S. and other states to fund new ventures in Sierra Leone, East Timor, the Congo and elsewhere. Finally, Annan restored the status of moral arbiter — of "secular pope" — with which Hammarskjold had first imbued the office.

The U.N. Annan inherited was a protector of states and their prerogatives; the one he wishes, somewhat forlornly, to leave behind would be a protector of individuals — even against the state. Perhaps the most resonant words he has spoken during his decade in office came during his 1999 address to the General Assembly, when he defined the "core challenge" of the institution as ensuring that "massive and systematic violations of human rights — wherever they may take place — should not be allowed to stand."

Annan's support for the doctrine of "humanitarian intervention" was praised in the West and condemned in the developing world, where it was seen as a license to violate sovereignty. The issue became unavoidable, and in its 60th anniversary meeting last year, the General Assembly formally accepted the principle that states had a responsibility to protect their own citizens and a duty to intervene to stop extreme cases of abuse in other countries.

To which, of course, one might say: Words are cheap. Indeed, Darfur's endless agonies seem to have been scheduled to expose the gross hypocrisy of "the international community." No country, including the U.S., which has been in the forefront of efforts to respond to the atrocities, has suggested sending troops against the will of the Sudanese government — the ultimate "responsibility" entailed in the responsibility to protect. But Annan, who as head of U.N. peacekeeping in 1994 was silent about the growing threat in Rwanda, has been a voice in the wilderness when it comes to Darfur.

In the end, the U.N. may be institutionally incapable of meeting this core challenge, in which case Annan's perverse achievement has been to demonstrate the limits of the institution to which he has devoted his life.

James Traub's latest book, "The Best Intentions: Kofi Annan and the U.N. in the Era of American World Power," will be published in October. He wrote this commentary for the Los Angeles Times.