The American attack upon Syria has been called illegal, uncontitutional and beyond the power of the Presidency. This is wrong and I will explain to you why.
The Congress passed the War Power Resolution after the Bay of Tonka incident. It is called an incident as even today there are question as to what exactly happened that night and if the US Navy was, in fact, attacked. The question and answer need not be addressed here. Congress did not like being dragged into a war without their approval. So then we jump to this attack, undertaken by the USA, UK and France. Was this an illegal action on the part of the President?

Even with the War Powers Resolution of 1973, it is generally agreed, that the commander-in-chief role gives the President power to repel attacks against the United States. The question is this strike such an act? When did Syria attack the United States? They did when Syria used poison gas to kill civilians, the Geneva Protocol, (1925) the 1972 Biological Weapons Convention (BWC) and the 1993 Chemical Weapons Convention (CWC). Whatever agent it is determined to be. As of this writing it is reported to be chlorine gas, a banded weapon used on the battlefields of World War One. Also, the 1998 Rome Statute of the International Criminal Court Makes it a war crime to employ chemical weapons in international conflicts. (2010 amendment extends prohibition to internal conflicts.) Thereby making this attack also a war crime subject to arrest by the international court.But even so, what does this mean for the USA in specific of the attack. What gives the President the power to attack Syria in the first place?
U.S. Constitution, Article VI: “This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land…” (Note that the USA is treaty member listed above.)
“And the judges in every state shall be bound thereby…”
That rule of law kicks in here, no judge can deny the obligation of any formal treaty entered into by the US government.
“…and all executive and judicial officers, both of the United States and of the several states, shall be bound by oath or affirmation, to support this Constitution; …”
Kind of obvious, but needing to be said, you took an oath for your position in government, you are bound by this oath to support the Constitution and thereby by adoption, all treaties as well.
As such, a violation of said treaty, against use of chemical weapons, that has been officially approved by the Senate, and has becomes part of American law, must be protected. The Executive, aka The Office of President, must therefore protect the law of the several state and the rule of law by protecting the treaty as it was any other part of the Unitied States. You cannot decide to not protect Montana or ignore those parts of the criminal code you do not personally agree with. By your oath or affirmation, you are duty bound to uphold the treaty.
Thus by international law, International Treaty law, American law, and the American Constitution, the USA must protect the treaties it has signed as if they were a part of the United States. This includes the one against attacking civilians with chlorine gas or nerve agents. Thus Syria attacked the USA by defying the treaty against said use of controlled weapons, just as if the attack was on downtown New York.