WASHINGTON — President Trump ordered the military on Thursday to carry out a missile attack on Syrian forces for using chemical weapons against civilians. The unilateral attack lacked authorization from Congress or from the United Nations Security Council, raising the question of whether he had legal authority to commit the act of war.

Mr. Trump and top members of his administration initially justified the operation as a punishment for Syria’s violating the ban on chemical weapons and an attempt at deterrence. But they did not make clear whether that was a legal argument or just a policy rationale.

The strike raises two sets of legal issues. One involves international law and when it is lawful for any nation to attack another. The other involves domestic law and who gets to decide — the president or Congress — whether the United States should attack another country.

Did Trump have clear authority under international law to attack Syria?

No. The United Nations Charter, a treaty the United States has ratified, recognizes two justifications for using force on another country’s soil without its consent: the permission of the Security Council or a self-defense claim. In the case of Syria, the United Nations did not approve the strike, and the Defense Department justified it as “intended to deter the regime from using chemical weapons again,” which is not self-defense.