"Under the Constitution, the war powers are divided between Congress and the President. Among other relevant grants, Congress has the power to declare war and raise and support the armed forces (Article I, Section 8), while the President is Commander in Chief (Article II, Section 2). It is generally agreed that the Commander-in-Chief role gives the President power to utilize the armed forces to repel attacks against the United States, but there has long been controversy over whether he is constitutionally authorized to send forces into hostile situations abroad without a declaration of war or other congressional authorization. [...] This report examines the provisions of the War Powers Resolution, actual experience in its use from its enactment in 1973 through March 2015, and proposed amendments to it. Appendix A lists instances which Presidents have reported to Congress under the War Powers Resolution, and Appendix B lists certain instances of the use of U.S. armed forces that were not reported."
CRS Report for Congress, R42699
Congressional Research Service: https://crsreports.congress.gov/