Final answer:
The document that directs the deployment and/or employment of U.S. forces, issued by the Secretary of Defense, is based on executive orders from the President, who acts as the Commander in Chief of the armed forces and has the authority to manage military operations and the Militia when federalized.
Step-by-step explanation:
The document issued by the authority and direction of the Secretary of Defense (SecDef) that directs the deployment and/or employment of forces is an executive order or directive. This kind of order originates from the President of the United States in their capacity as the Commander in Chief of the U.S. armed forces. The President oversees all elements of the U.S. armed forces, including the Army, Navy, Marine Corps, Air Force, and the Militia of the several States when called into service.
Additionally, the President has the military power to call into service and federalize the National Guard in times of war or national emergency. According to the U.S. Constitution and subsequent congressional resolutions, the President is empowered to take necessary measures to repel attacks against U.S. forces and to prevent further aggression, reaffirming the role of the President in military decision-making and force deployment.