The authors of the Constitution were explicit on this point. As James Madison noted, “In no part of the Constitution is more wisdom to be found, than in the clause which confides the question of war or peace to the legislature, and not to the executive department. . . . Those who are to conduct a war cannot in the nature of things, be proper or safe judges whether a war ought to be commenced, continued, or concluded.” Alexander Hamilton wrote in The Federalist that there was a clear distinction between the U.S. president’s authority as commander-in-chief, which involved “nothing more than the supreme command and direction of the military and naval forces” and that of the British king, who could declare war unilaterally.
~ John Fund, Obama Embraces the Imperial Presidency
To which I will add that the quote from James Madison is not taken out of context. You can read it for yourself, in letter number 4 of his “Letters of Helvidius.
You can also learn a metric ton of useful stuff about our nation, it’s birth and our constitution by sitting down with a copy of The Federalist. Which is a series of essays by Alexander Hamilton, John Jay, and James Madison.