From the Document: "Recent U.S. airstrikes in Iraq and Syria have raised legal questions concerning the scope of the President's power to use force against Iran and Iran-backed organizations. In late December, U.S. forces conducted airstrikes against five facilities in Iraq and Syria used by Kata'ib Hizbollah--an entity with ties to Iran designated by the Department of State as a foreign terrorist organization. According to a Department of Defense (DOD) statement, the strikes came in response to attacks on military bases in Iraq that host U.S. forces engaged in the campaign to defeat the Islamic State. [...] 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/