UCMJ - Uniform Code of Military Justice

"In this case . . . the President has acted in a field with a history of congressional participation and regulation.  In the Uniform Code of Military Justice (UCMJ), 10 U.S.C. § 801 et seq., which Congress enacted, building on earlier statutes, in 1950 . . . and later amended . . . Congress has set forth governing principles for military courts."

"The UCMJ as a whole establishes an intricate system of military justice.  It authorizes courts-martial in various forms, 10 U.S.C. §§ 816-820; it regulates the organization and procedure of those courts, e.g., §§ 822-835, 851-854; it defines offenses, §§ 877-934, and rights for the accused, e.g., §§ 827(b)-(c), 831, 844, 846, 855; and it provides mechanisms for appellate review, §§ 859-876b." 

"As explained below, the statute further recognizes that special military commissions may be convened to try war crimes.  While these laws provide authority for certain forms of military courts, they also impose limitations [on the President of the United States], at least two of which control this case."