ON March 17, 1969, President Richard M. Nixon began a secret bombing campaign in Cambodia, sending B-52 bombers over the border from South Vietnam. This episode, largely buried in history, resurfaced recently in an unexpected place: the Obama administration’s “white paper” justifying targeted killings of Americans suspected of involvement in terrorism.

President Obama is reportedly considering moving control of the drone program from the Central Intelligence Agency to the Defense Department, as questions about the program’s legality continue to be asked. But this shift would do nothing to confer legitimacy to the drone strikes. The legitimacy problem comes from the secrecy itself — not which entity secretly does the killing. Secrecy has been used to hide presidential overreach — as the Cambodia example shows.

On Page 4 of the unclassified 16-page “white paper,” Justice Department lawyers tried to refute the argument that international law does not support extending armed conflict outside a battlefield. They cited as historical authority a speech given May 28, 1970, by John R. Stevenson, then the top lawyer for the State Department, following the United States’ invasion of Cambodia....