The President has vast Article II powers to conduct war. But that power isn’t limitless. The best explanation of that limit is found in Federalist 69 written by Alexander Hamilton; it’s also explained well more recently in the dissent written by Justice Scalia and joined by Justice Stevens in Hamdi v. Rumsfeld. That dissent states that the commander in chief clause makes the President the top general whenever Congress starts wars.
Here’s the relevant passage written by Scalia in the Hamdi dissent:
Congress’s authority “[t]o raise and support Armies” was hedged with the proviso that “no Appropriation of Money to that Use shall be for a longer Term than two Years.” U.S. Const., Art. 1, §8, cl. 12. Except for the actual command of military forces, all authorization for their maintenance and all explicit authorization for their use is placed in the control of Congress under Article I, rather than the President under Article II. As Hamilton explained, the President’s military authority would be “much inferior” to that of the British King:
“It would amount to nothing more than the supreme command and direction of the military and naval forces, as first general and admiral of the confederacy: while that of the British king extends to the declaring of war, and to the raising and regulating of fleets and armies; all which, by the constitution under consideration, would appertain to the legislature.” The Federalist No. 69, p. 357.
When Congress authorizes a war, the President becomes its top prosecutor. Except when the country is facing imminent attack, the President cannot start wars without statutory authorization. And his power is still somewhat limited if in fact he does act unilaterally. The War Powers Resolution forbids the military to be in a hostile area for over 60 days without congressional authorization.
And here is the relevant text in the War Powers Resolution on when the President may conduct war:
The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to
(1) a declaration of war,
(2) specific statutory authorization, or
(3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.