Article 1, Section 8, Clause 14: Regulation of the military

To make Rules for the Government and Regulation of the land and naval Forces;

This follows naturally from the previous two clauses. It is the Congress, not the President as commander-in-chief, who promulgates regulations for the armed forces.

The importance of these cannot be overemphasized. An army is not just a mob of men who kill people and break things. It is ideally a highly disciplined organization that applies precisely the amount of force it is ordered to in order to defend the country from invasion or insurrection or to advance national policies, when necessary, by force. It is leadership according to regulations that transforms a mob into an army.

During the Revolutionary War, Congress established some 69 Articles of War to govern the (then) Continental Army. A  new set of Articles of War, governing both the army and navy, were enacted in 1806. They have only infrequently been revisited since then, the most recent major revision being the Uniform Code of Military Justice of 1951. Though significantly revised, this remains in effect today.