Barack Obama has turned drones into his signature counterterrorism tool, even personally selecting targets from a “kill list” as he has deployed this new sort of air force to rain death down on terrorists across two continents and bludgeon al Qaeda into submission. But far too much of the U.S. president’s secret assassination program has been shielded from legal scrutiny — and Jameel Jaffer, an influential lawyer for the American Civil Liberties Union specializing in national security issues, is working to change that.