One day before President Obama is due to deliver a major speech on national security, his administration on Wednesday formally acknowledged that the United States had killed four American citizens in drone strikes in Yemen and Pakistan. In a letter to Congressional leaders obtained by The New York Times, Attorney General Eric H. Holder Jr. disclosed that the administration had deliberately killed Anwar al-Awlaki, a radical Muslim cleric who was killed in a drone strike in September 2011 in Yemen. The American responsibility for Mr. Awlaki’s death has been widely reported, but the administration had until now refused to confirm...