Ten years ago this month, senior United Nations officials were hard at work equivocating over a crisis. A cynic might say that the U.N. exists in a constant state of equivocation. But in March 2009, its leaders were mired in an especially grim political mess—and handling it badly.

The cause of their troubles lay in northern Sri Lanka. After decades of civil war, the Sri Lankan military was carrying out a final offensive against the Liberation Tigers of Tamil Eelam, a rebel group with a long history of atrocities. As the decisive battle wore on, U.N. officials and journalists in the war zone reported that the army was also behaving brutally, killing thousands of civilians. ...