He threatened to veto it because it threatened his power. The Obama administration claimed that the Authorization for Use of Military Force Against Terrorists A

He threatened to veto it because it threatened his power. The Obama administration claimed that the Authorization for Use of Military Force Against Terrorists Act of 2001 already gave him the authority to detain citizens, and that the original act confused that (by requiring military custody). There is a section that says that the requirement for military custody does not apply to US citizens, but that does not mean that US citizens cannot be detained by the President in civilian custody. This is the section that a lot of people are getting hung up on and saying, "Well, it says right here that US citizens can't be detained." No, it says that US citizens aren't required to be held in mandatory military custody.