Updated: Recent U.S. Airstrikes: Legal Authorities and Questions [Updated February 18, 2020] [open pdf - 778KB]
From the Document: "'The Trump Administration has reported to Congress a 'change in application of the existing legal and policy frameworks' regarding the use of military force, which presented the following legal justification for the January airstrikes: Article II of the United States Constitution, empowers the President, as Commander in Chief, to direct the use of military force to protect the Nation from an attack or threat of imminent attack and to protect important national interests. Article II thus authorized the President to use force against forces of Iran, a state responsible for conducting and directing attacks against United States forces in the region.' [...] This Sidebar examines the President's domestic legal authority to initiate military action under the two sources cited in the National Security Advisor's statement: the Constitution and the 2002 authorization for use of military force related to Iraq (2002 AUMF). Although not discussed in O'Brien's statement, this Sidebar also examines the implications of the ban on assassinations in Executive Order 12333 as well as the international legal framework governing the strikes."
CRS Legal Sidebar, LSB10391
Congressional Research Service: https://crsreports.congress.gov/