The “arguments” being put forward by U.S. officials and others to justify an attack on Syria are as ludicrous as you would expect :
At the House hearing, Democrats grilled Mattis on the wisdom and legality of Trump ordering an attack on Syria without explicit authorization from Congress. Mattis argued it would be justified as an act of self-defense, with 2,000 U.S. ground troops in Syria [bold mine-DL]; he insisted he could not talk about military plans because an attack “is not yet in the offing.”
Mattis’ claim is absurd, and I said as much yesterday:
That's a new one. Our illegal military presence somehow justifies our illegal attack https://t.co/7ycXXcUa1E 
— Daniel Larison (@DanielLarison) April 12, 2018 
The U.S. military presence in Syria is already illegal in a couple ways. The Syrian government has never given the U.S. permission to operate on its territory, and Congress has never authorized any mission inside Syria. Our forces have no international mandate to be there, and they certainly don’t have a mandate to fight the Syrian government. Obama had no legal authority to expand the war on ISIS into Syria, and so he had no authority to send U.S. forces into Syria as part of that war. Even if the presence of U.S. forces in Syria had been authorized by Congress, it would still be in violation of international law. Even if the war on ISIS had been authorized, it would give the U.S. no right to initiate hostilities against the Syrian government.
The fact that the U.S. has an illegal military presence in Syria obviously doesn’t give our government license to attack the Syrian government or its allies, and it just drives home that all U.S. military operations inside Syria are illegal. The U.S. cannot use its other illegal behavior to justify a new attack, and the fact that Mattis is reduced to making this claim shows that there is no credible case to be made that attacking the Syrian government is legal.
Noted torture enabler John Yoo has never seen an illegal government action he couldn’t rationalize, so of course he thinks  attacking the Syrian government is fine:
“I think that statutory authorization comes from the [authorization for use of military force] passed after 9/11, which allows the use of force with regard to any group connected to the 9/11 attacks, which includes ISIS (which is an offshoot of al Qaida). Because ISIS is operating in Syria, the U.S. can use force in Syria,” Yoo said in an email.
Perhaps this is where Paul Ryan  gets his nonsensical belief that the 2001 AUMF covers an attack on the Syrian government. It shouldn’t have to be said, but Yoo is absolutely wrong. The 2001 AUMF cannot possibly apply to the Syrian government, which is not in league with Al Qaeda or any other jihadist groups. If you don’t like Yoo’s first garbage opinion, he has another:
An unrepealed 2002 war authorization allowing for the invasion of Iraq to topple Saddam Hussein also can justify an attack on Assad, Yoo said, because that legislation “allows the use of force to stabilize Iraq, which the use of force against Assad could advance.”
This is such a stupid and dishonest misreading of the 2002 AUMF that it shouldn’t need to be refuted. To be clear, an authorization for using force against the old Iraqi government cannot be used to justify attacks on a different government in a different country over 15 years later. Of course, Yoo doesn’t think there is any need to bother with these torturous misinterpretations because he believes the president has inherent power to start wars anywhere whenever he wants:
“But my constitutional view is that the president does not need these statutory authorizations because as commander-in-chief Trump can deploy the armed forces on his own…”
This is what is at stake in the debate over attacking Syria. If you don’t think attacking the Syrian government is illegal, you are reduced to buying into the anti-constitutional nonsense of John Yoo and others like him. If you think that the president’s authority is limited by the Constitution and the power to decide when and where to go to war rests solely with Congress, you have to oppose an attack on the Syrian government as the illegal overreach that it is.