Just over two years ago, he became legal adviser to Hillary Rodham Clinton’s State Department, and in that job, he has become the administration’s defender of the right to stay engaged in a conflict against Libya without Congressional approval. He argues that the president can proceed because the country is not actually engaging in “hostilities.” Because “hostilities” is “an ambiguous standard,” he has argued, the president need not withdraw forces to meet the resolution’s requirement of an automatic pull-out, 60 days after “hostilities” begin, absent express Congressional approval for the war. The conflict is in its fourth month, and no such consent has been given.

Mr. Koh’s allies, speaking more in sorrow than in anger, are mystified and disheartened to see their hero engaging in legalistic “word play.” To them, it’s as if he has torn off his team jersey, midgame, and put on the other side’s. Mary Ellen O’Connell, a Notre Dame law professor who has known Mr. Koh for a quarter-century, is seeking an answer to this question: “Where is the Harold Koh I worked with to ensure that international law, human rights and the Constitution were honored during the Bush years?”…

In our conversations, he took pains to try to reconcile his current and past positions on war powers by pointing to Libya as a particularly complicated case. The action was launched, he noted, to prevent a massacre of rebellious Libyans at the brutal hands of Col. Muammar el-Qaddafi — and even though Mr. Obama did not first go to Congress, the United Nations Security Council authorized the intervention for the purpose of protecting civilians. Citing his convictions as a human rights activist, he suggested that there was an inescapable tension “between preventing atrocities and staying true to the letter and spirit of the War Powers Resolution.”