The Congo has long had a knack for bringing out the worst in foreign adventurers, from King Leopold II of Belgium to Joseph Conrad's fictional Kurtz. Now it is forcing a well-intentioned visitor, the United Nations, to reconsider how it keeps the peace in time of war.

The U.N. faced harsh criticism in the mid-1990s when its peacekeepers stood aside as atrocities unfolded in Rwanda and in Srebrenica during the Bosnian war. Determined not to repeat such failures, the U.N. resolved...