Who shall have the power to "be Commander in chief of the Army and Navy of the United States, and of the Militia of the several states," according to the US Constitution?
The President of the USA
Article II, Section 2 of the Constitution states, in part:
The President shall be commander in chief of the Army and Navy of the United States, and of the militia of the several states, when called into the actual service of the United States;
This does not mean that only the President has responsibilities under the Constitution concerning the armed forces. The Congress, under Article I, Section 8 states that the Congress is responsible for:
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;
To provide for calling forth the militia to execute the laws of the union, suppress insurrections and repel invasions;
To provide for organizing, arming, and disciplining, the militia, and for governing such part of them as may be employed in the service of the United States, reserving to the states respectively, the appointment of the officers, and the authority of training the militia according to the discipline prescribed by Congress;
And there's the rub - Congress is supposed to raise and maintain the armed forces and declare when they'll be used and it's the President, once Congress has declared they shall be used, who determines how they will be used.
Both sides have tried to exercise more and more control over the use of the military over the years. Presidents have sent military forces overseas without Congressional approval (the Vietnam conflict), with Congress then passing the War Powers Act (which forces the President to report to Congress within 48 hours when any military forces are used), and now with the threat of terrorism on the rise and the lack of a party to declare war on, Presidents are using military forces in a war-like way but without having declared war.