KOFI ANNAN TURNED in his resignation as United Nations special envoy to Syria on Thursday, but his mission was over months ago. It was doomed by Syrian President Bashar al-Assad, who was never serious about peace and determined to crush the opposition, and by his chief backer, Russian President Vladimir Putin. The five months that Mr. Annan devoted to talk, with the ill-considered backing of the Obama administration, simply gave Mr. Assad more time to wage war.

The failed mission offers vital lessons for the future. The first is one that Mr. Annan should know well: He learned it in the 1990s conflict in the former Yugoslavia. Then as now, the leading powers were initially hesitant to use military force. Then as now, U.N. personnel were sent to a battlefield and proved ineffective in the face of evil. After the worst massacre in postwar European history, by Bosnian Serbs at Srebrenica, Mr. Annan wrote a searing retrospective for the United Nations. He declared that “when peacekeeping operations are used as a substitute for. . . political consensus they are likely to fail.” He added, “The job simply cannot be done.”

These words are just as valid today as when Mr. Annan wrote them in 1999. The U.N. Security Council sent unarmed monitors into an intensifying war zone in Syria without a consensus of the leading powers to back them up. It was wishful thinking to believe that Mr. Assad would be coaxed into retirement by a divided council. For way too long, the Obama administration — eager to avoid more decisive action — clung to unrealistic hopes that Annan’s plan would work.

The administration also seems to have misjudged how steadfastly Mr. Putin would stand behind the Syrian regime. There were vague hopes that, sooner or later, the Kremlin would give up on Mr. Assad. But Mr. Putin refused to budge. For years, he has been disdainful and fearful of the “color” revolutions — Orange in Ukraine, Rose in Georgia — that swept autocrats from power. In recent months, he has heard the footsteps of protest outside his own Kremlin walls. He was not about to applaud the drumbeat of another revolution seeking to topple a dictator in Syria. It’s not comforting to see Mr. Putin express an outmoded, Cold War mind-set that sees Russia’s interest in opposing the United States at every step — but it is something that should not have surprised the White House.

Many in Washington misread Mr. Assad. In some early accounts, he was portrayed as a moderate or modernizing figure, a more enlightened version of his brutal father, Hafez al-Assad, who ruled Syria for three decades. But the events of the past 18 months suggest that the younger Mr. Assad has matched his father’s record of despotism: He is willing to slaughter an unlimited number of his own people in order to cling to power. Now that diplomacy has utterly failed to stop him, it is time for the Obama administration to consider measures that stand a real chance of accelerating his downfall — beginning with greater material support for the opposition.