Defense Primer: Legal Authorities for the Use of Military Forces [Updated January 3, 2020] [open pdf - 438KB]
From the Document: "By the Framers' apparent design, in order to keep the nation's 'purse' and the 'sword' in separate hands and in other ways hinder its embroilment in unnecessary wars, the Constitution divides war powers between Congress and the President. Congress is empowered to declare war, provide for and regulate the Armed Forces, and issue letters of marque and reprisal, as well as to call forth the militia to suppress an insurrection, repel an invasion, or 'execute the Laws of the Union.' The President, as the Commander in Chief, has the responsibility to direct the Armed Forces as they conduct hostilities, put down insurrections, or execute the law when constitutionally authorized to do so. But the extent to which the President has independent authority under the Constitution, without explicit statutory support, to use the military for purposes other than to repel a sudden attack is the subject of long-standing debate."
CRS In Focus, IF10539
Congressional Research Service: https://crsreports.congress.gov/