Article I, Section 8

Let's review.

The Congress shall have power ... [among many other things]

To define and punish piracies and felonies committed on the high seas, and offenses against the law of nations;

To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water;

To raise and support armies, but no appropriation of money to that use shall be for a longer term than two years;

To provide and maintain a navy;

To make rules for the government and regulation of the land and naval forces; ...

--And

To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof. [emphasis mine]

Commander-in-Chief is all well and good, but Congress gets to make the rules for the regulation of the land and naval forces, and, if that weren't completely clear, all other powers the Constitution gives the government. The President gets to take action, but Congress get to tell him what he can do.

Those silly founders. Why were they so vague?

(thanks to The Editors at The Poor Man for reminding me.)