There is no “big idea” easier to pay homage to in principle, or harder to make work in practice, than the peacekeeping role of the United Nations. This is painfully clear in a new memoir by Kofi Annan, its former secretary-general.

The latest failure of the U.N. dream was Annan’s mediating mission to Syria. For months, he tried to cajole President Bashar al-Assad into stopping the killing and starting a political transition that would avert civil war. To which he received the standard answer to well-meaning U.N. missions: Go away. You cannot stop me.

Annan finally did walk away last month, ending his Syria mission and probably his career as a mediator. What will come next, it’s increasingly clear, is a paramilitary covert action, supported by the U.S. and most of its allies, to help the Syrian rebels accomplish what the U.N. could not.

Annan’s new memoir, “Interventions,” is a study in the failure of a noble idea. And it should cause readers to reflect why, in so many cases, the international community has been unable to gather sufficient force (or will) to prevent conflict. Another failure is probably ahead with Iran, where six years of escalating U.N. sanctions have not curbed Tehran’s nuclear program, and unilateral military intervention is increasingly likely.

The U.N. today is bootless; the will of most members for a change of government in Syria, for example, is too easily blocked by the veto of a single permanent Security Council member, such as Russia.

Annan gives an honest, devastating account of some of the U.N.’s errors during his decades with the organization, especially in his description of the peacekeeping missions in the 1990s in Somalia, Rwanda and Bosnia, which he collectively describes as the organization’s “greatest of failures.”

Somalia was a project of Annan’s predecessor, Boutros Boutros-Ghali. A U.N. peacekeeping force known as UNOSOM II had been authorized in March 1993, described by Madeleine Albright, then U.N. ambassador, as “an unprecedented enterprise aimed at nothing less than the restoration of an entire country.”

But the American military contribution was restricted to a small special operations force hunting the rebel Gen. Mohamed Aidid; it communicated with Boutros-Ghali and didn’t coordinate with the rest of the U.N. force. When the Americans got slaughtered in a bloody ambush in Mogadishu (depicted unforgettably in the film “Black Hawk Down”), Washington bailed out and UNOSOM II quickly collapsed.

The Somalia mess made the U.N. so nervous about intervention that it ignored an appeal a few months later from its own representative in Rwanda that a genocidal massacre was about to begin there.

In January 1994, Gen. Romeo Dallaire, the French Canadian commander of a small force called UNAMIR, cabled New York that the Hutu-led government in Kigali was planning the “extermination” of Tutsis. He concluded his message, “Allons-y.” Let’s go.

The U.N. did nothing. Three months later, 800,000 Rwandans were dead.

The third debacle was Bosnia. In April 1993, the Security Council demanded that the town of Srebenica, filled with 60,000 Muslim refugees and encircled by Bosnian Serb forces, become a “safe area … free from armed attacks.” The refugees waited more than two years for the U.N. to deliver.

In July 1995, Gen. Ratko Mladic committed his infamous massacre. A month later, UNPROFOR finally intervened.

When Annan became secretary-general, the U.N. tried to bolster its peacekeeping efforts. It did better in East Timor, Kosovo and Libya in putting some teeth in the concept of a “responsibility to protect.” But the story has been the U.N.’s limitations — in dealing with Iraq, the Palestinian issue, Iran and now Syria.

What to do? Albright and 15 other former foreign ministers just sent a letter to President Vladimir Putin saying they were “gravely disappointed” by Russia’s failure to support the U.N. mission in Syria. Albright’s office says the Russians responded negatively.

As the whole of this revealing book demonstrates, there’s got to be a better way to prevent ruinous conflicts.

David Ignatius writes about foreign affairs for The Washington Post. He can be contacted at:

davidignatius@washpost.com.