"In his letter to Congress under the War Powers Resolution, President Trump cited his Article II powers as his domestic legal authority for the April 6 missile strike on Al Shayrat airbase in Syria, but did not describe a legal basis to justify the action under international law. The missile strike may raise questions under international law including, in particular, whether the action is consistent with U.S. obligations under the U.N. Charter. The answer may in turn raise questions about whether the validity of the international legal basis has any bearing on the validity of the action pursuant to U.S. domestic law. This Sidebar provides a brief overview."
CRS Legal Sidebar, April 17, 2017
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html