Suppose a future president—let's call him Mitt Romney—declares that last fall's killing of al Qaeda leader (and American citizen) Anwar al-Awlaki amounted to an "assassination." He orders a special prosecutor to pursue everyone from the drone pilot who pulled the trigger all the way to Defense Secretary Leon Panetta and CIA Director David Petraeus. The murder investigation triggers lawsuits by Awlaki's family, litigated gratis by law schools, human-rights groups and their legal allies, whose leaders the president later rewards with plum jobs.

Reverse the political polarities, and you have the counterterrorism policies of President Barack Obama. And that is the past—and the future—that I hope our nation escaped this week with the Ninth Circuit Court of Appeals decision in Padilla v. Yoo. That Yoo is me.

The three-judge panel unanimously agreed that José Padilla, an American al Qaeda operative convicted and sentenced in 2007 for running U.S. terrorist cells, could not sue me for my legal advice that the Constitution allowed his earlier detention as an enemy combatant. Padilla also threw in imaginary claims that he was tortured in U.S. Navy custody.

John Yoo is a visiting scholar at AEI.

The full text of this article is available vis subscription to The Wall Street Journal. It will be posted to AEI.org on Monday, May 7.