Defense Primer: President's Constitutional Authority with Regard to the Armed Forces [Updated December 29, 2021] [open pdf - 705KB]
From the Document: "The Constitution makes the President Commander in Chief of the Armed Forces, but does not define exactly what powers he may exercise in that role. Nor does it explain the extent to which Congress, using its own constitutional powers, may influence how the President commands the Armed Forces. Separation-of-powers debates arise with some frequency regarding the exercise of military powers. Early in the nation's history, Alexander Hamilton wrote in 'The Federalist', No. 69, that the Commander in Chief power is 'nothing more than the supreme command and direction of the military and naval forces, as first general and admiral of the confederacy.' Concurring in that view in 1850, the Supreme Court in 'Fleming v. Page' stated, '[The President's] duty and his power are purely military. As Commander in Chief, he is authorized to direct the movements of the naval and military forces placed by law at his command, and to employ them in the manner he may deem most effectual to harass and conquer and subdue the enemy."
CRS In Focus, IF10534
Congressional Research Service: https://crsreports.congress.gov/