Defense Primer: President's Constitutional Authority with Regard to the Armed Forces [Updated March 11, 2020] [open pdf - 609KB]
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. [...] Under the Trump Administration's apparent view, such use of force does not require congressional consultation or authorization, seemingly without consideration of the likelihood of escalation into 'war in the constitutional sense.' The report also cited the 2002 authorization for the use of military force against Iraq in support of the operation."
CRS In Focus, IF10534
Congressional Research Service: https://crsreports.congress.gov/