“KILLING AWLAKI was illegal, immoral and dangerous,” read the headline of an essay by University of Notre Dame Law School professor Mary Ellen O’Connell. The piece, published over the weekend on CNN.com, criticized the Sept. 30 drone strike in Yemen that took the life of two U.S. citizens affiliated with al-Qaeda, including Anwar al-Awlaki, the radical Muslim cleric who was a leader of al-Qaeda in the Arabian Peninsula.

“The president and his officials know that it is unlawful to kill persons in this way outside of armed conflict hostilities,” asserted Ms. O’Connell, a specialist in international law.

We disagree with this claim of illegality, but it is not easily or intuitively dismissed. After all, should not all American citizens be afforded due process of law before being summarily executed, as some critics might put it? The administration should respond quickly and definitively, and there is no better way to do that than by making public a memorandum from the Justice Department’s Office of Legal Counsel (OLC) that lays out the specific legal grounds and standards for the attack.

The outlines of the administration’s justification for targeting an enemy outside of the battlefield are well known. In a speech last year, State Department legal adviser Harold Koh noted that the United States is at war with al-Qaeda and its affiliates and that Congress authorized the use of military force against these enemies. He also cited the right of national self-defense under international law, which allows a country to strike at enemies who pose an imminent threat. Such strikes may be carried out beyond a recognized war zone if the enemy is hiding in a country that is unwilling or unable to take action. John Brennan, a senior counterterrorism adviser to the president, reiterated these principles last month in a speech at Harvard Law School. But neither man delved into specific legal authorities nor addressed the legality of targeting an American citizen.

Releasing the OLC opinion would shed light on the issues that touch on such a strike, including how the administration balanced the due-process rights of U.S. citizens with Mr. Awlaki’s decision to become a belligerent. The appropriate congressional overseers should be given the opportunity to read the opinion in a classified setting; the administration could redact portions of the memo that deal with sensitive national security information, including sources and methods, and then release the memo to the public.

Refusal to do so could fuel inaccurate criticism that this president is acting no differently than his predecessor, who justified dubious enhanced-interrogation techniques through secret and now discredited “torture memos.” The Bush administration used the deeply flawed torture memos as a pretext to thumb its nose at domestic laws and international strictures that prohibit cruel and inhumane treatment. Mr. Obama is acting in sync with international law in defending the country against an enemy belligerent who forfeited constitutional protections by directing hostile forces against his homeland.