US Constitution, Article I, Section 8: Powers of Congress:
Enumerated powers
The Congress shall have power. ... .
To lay and collect Taxes, Duties, Imposts and Excises. ... .
To define and punish Piracies and Felonies committed on the high Seas, and Offenses against the Law of Nation. ... .
Congress can declare war, but it is the President, as Commander-in-Chief, who must prosecute the war, via the armed forces. Similarly, the Congress sets the laws and penalties against piracy- and violations of the Law of Nations- but it is the Chief Executive who will direct the action required thereof, as, for instance, ordering the launching of several dozen TLAM missiles from Navy ships.
The violations of the Law of Nations in this case would be The Geneva Gas Protocol and Common Article 3 of the Geneva Convention, the grounds for the conviction and hanging of one Saddam Hussein.
If it is Congress that is charged with enumerated power to "punish" violations of the laws of nations, then any use of military force to punish a violator would necessarily require a resolution by congress directing the manner of punishment. Read that section again carefully.
I think it makes specific congressional approval of this kind of action mandatory and not discretionary.