Separation of powers under the United States Constitution
The president is the civilian Commander in Chief of the Army and Navy of the United States. He has the authority to command them to take appropriate military action in the event of a sudden crisis. However, only the Congress is explicitly granted the power to declare war per se, as well as to raise, fund and maintain the armed forces. Congress also has the duty and authority to prescribe the laws and regulations under which the armed forces operate, such as the Uniform Code of Military Justice, and requires that all Generals and Admirals appointed by the president be confirmed by a majority vote of the Senate before they can assume their office.

Q: Who is the commander and chief of the Army?
The president