Defense Primer: President's Constitutional Authority with Regard to the Armed Forces [December 17, 2018] [open pdf - 497KB]
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."
CRS In Focus, IF10534
Congressional Research Service: https://crsreports.congress.gov/