President Trump has been warning that missiles “will be coming” against Syria’s government, which he accused of killing civilians with chemical weapons in the final rebel-held town of Douma near Damascus, although it is not clear when. But some legal experts warn an attack would be illegal.

Several scholars say the Constitution doesn’t allow Trump to attack Syria’s military because it did not first attack the U.S. or its troops, which would allow Trump to act without congressional authorization for 60 days under the War Powers Resolution of 1973.