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The US Constitution has placed no limit upon the war powers of the government, but they are regulated and limited by the laws of war. One of these powers is the right to institute military governments.

Military government foreign territory
The erection of such governments over the persons and territory of a public enemy is an act of war; is in fact the exercise of hostilities without the use of unnecessary force. It derives its authority from the customs of war, and not the municipal law. It is a mode of retaining a conquest, of exercising supervision over an unfriendly population, and of subjecting malcontent non-combatants to the will of a superior force, so as to prevent them from engaging in hostilities, or inciting insurrections or breaches of the peace, or from giving aid and comfort to the enemy.