Paul Starobin is the author of “After America: Narratives for the Next Global Age.”

HAROLD HONGJU KOH, the former dean of the Yale Law School, has been one of the country’s foremost defenders of the notion that the president of the United States can’t wage wars without the approval of Congress. During the Bush administration, he was legendary for his piercing criticisms of “executive muscle flexing” in the White House’s pursuit of the so-called war on terror.

Even more, he was described by those who knew him as the inspiration for a generation of human rights activists and lawyers passionately committed to a vision of a post-imperial America as a model of constitutional restraint. His colleagues viewed him as not only a brilliant scholar but a “liberal icon.”

Suddenly, though, Mr. Koh seems to be a different person.

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?”

For Mr. Koh, criticism from his old allies is wrenching. In an interview at his State Department office, decorated with memorabilia from his beloved Boston Red Sox, his voice thickened as he talked about colleagues from academia now taking sharp issue with him. “I love these people, many of them,” he said. He understands that his critics feel betrayed, but he feels misunderstood; his friends are failing to appreciate that the “Old Harold,” as they now call him, has a new job. “I am changing roles,” he said. “People’s lives have seasons.”

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.”

While this argument might justify an initial refusal by President Obama to go to Congress, given the imminent risk of massacres in Libya, it does not speak to his continuing refusal to do so.

So how can Congress still be sidestepped? Mr. Koh responds that the War Powers Resolution, passed by Congress in 1973, is imprecise on many particulars.

The question remains as to why Mr. Koh has labored so hard to come up with a position so at odds with his former self. One plausible explanation is that it was simply not his nature to take on his bosses in the Obama administration, who have no interest in going before a truculent Congress to seek approval of the Libya intervention. “He is not a lone wolf,” said Larry Lucchino, the president of the Boston Red Sox, a Yale Law graduate, and a friend.

“There’s a personality attribute Harold has — he is exceptionally loyal to institutions,” Mr. Lucchino said.

A mix of partisan and personal sympathies could be at work. Mr. Koh, a Harvard College and Harvard Law graduate, is a Democrat who owes his Washington rise to the Clintons, with Hillary having selected him for his current post and Bill, as president in the 1990s, having made him assistant secretary for democracy, human rights and labor at the State Department. Pressed on whether he felt pushed by Secretary Clinton and President Obama to take a view counter to his personal beliefs, he insisted that “I am not in a position where I feel I am choosing between my principles and my service.”

This is an especially sensitive matter for Mr. Koh: As he often relates with pride, his father, a diplomat for the democratic South Korean government, resigned and went into exile in the United States rather than stay on and work for the military junta that overthrew that regime in the 1960s. It’s also possible that Mr. Koh maneuvered himself into the awkward position he now occupies because of a miscalculation. The Libya intervention was supposed to last days, maybe a few weeks. It may never have occurred to him that he’d have to come up with a rationale for allowing Mr. Obama to proceed unimpeded by legislative shackles.

Whatever his motivations, it is sad to see Mr. Koh, with all his acumen, stretched out on a legal limb so long and so thin that one can almost hear it cracking. Talk, he suggested, to Laurence H. Tribe — the eminent constitutional law scholar whose course Mr. Koh took at Harvard Law. But no rescue came. “Harold is a good friend, one of my finest former law students,” Professor Tribe said in an e-mail, but “I disagree completely with his analysis of the War Powers Resolution.”

He is not winning support for his position in the academic world or even in some key parts of the administration. Legal advisers at both the Justice Department and Pentagon have declined to back the no-hostilities brief. One defender of this position, the Yale law professor Akhil Reed Amar, said that because “there are no body bags” of United States soldiers to this date, hostilities in Libya do not involve America.

But a common-sense definition of “hostilities” suggests they began back in March when American warplanes bombed the Qaddafi regime’s air-defense systems, noted Lori Fisler Damrosch, a Columbia law professor who in 1990 joined with Mr. Koh on a brief to compel the executive branch to share war powers with Congress.

The damage wrought by Mr. Koh’s volte-face extends well beyond his “liberal icon” reputation in the academy. A contorted reading of a law to suit a president’s political need can only invite further such contortions (and more public cynicism about Washington). And Mr. Koh’s opinion may set a worrisome precedent for the next war. His logic suggests that a war without United States boots on the ground can proceed indefinitely without Congressional approval. With drone warfare now expanding, with Mr. Koh’s approval, national-security decision-making stands to become the sole province of the executive.

In Mr. Koh’s 1990 book “The National Security Constitution,” which established his reputation as a bedrock believer in shared powers, he offered a lamentation on “why the president almost always wins in foreign affairs.” One reason, he said, was “Congressional acquiescence.”

Another, it seems, is that an imaginative executive-branch lawyer, in a pinch, can invent an argument that does the trick.