It has become necessary to make something perfectly and abundantly clear.
Members of the United Nations are, according to its Charter, to “refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state,,,,”  This is international law. The UN Charter is a treaty to which the United States is a party, so it is also the law of the United States. Attacking or even threatening to attack another sovereign nation is an unlawful act.
If the President wants to attack another nation that has not initiated military hostilities with the United States he must do two things. He must get authorization from Congress, , which he has now, at last, decided to do.  He must also bring the matter before the UN Security Council and seek a resolution authorizing the use of military force. It is doubtful the President will do that, since Russia will most certainly block any action by the Security Council that will involve a military strike against Syria. But if the United States launches an attack against Syria without authorization by the Security Council it will be in violation of international and domestic law. It really is that simple.
But haven’t the Syrians been using chemical weapons against the opposition forces? Isn’t that also an illegal action?
It would indeed be illegal for the Syrians to use chemical weapons, but it is not at all clear at present that it is the Syrian government that has been using them.  Even if it can be proven that the Syrian government has been engaging in such activity, that will not dispense with the legal requirements that the President must meet before he orders a military attack. Syrian President Bashar al-Assad won’t be winning the Nobel Peace Prize anytime soon, but any unlawful acts he is committing cannot justify the U.S. government in acting in kind.
Meanwhile, there is another voice to be heard.
Pope Francis has made a joint statement with King Abdullah II of Jordan that dialogue and negotiations are “the only option for putting an end to the conflict and violence” in Syria.  It should not be surprising that the Holy Father would take such a position. He is, after all, the guardian of Catholic tradition and doctrine, including the Just War Doctrine, which prohibits a nation from attacking another when it is not threatened. 
Archbishop Maroun Lahham, the patriarchal vicar for Jordan in the Latin Patriarchate of Jerusalem puts the matter more bluntly. “Our friends in the West and the United States have not been attacked by Syria,” he says. “With what legitimacy do they dare attack a country? Who appointed them as ‘policemen of democracy’ in the Middle East?”