It's Going To Be Hard To Find A Legal Justification For Attacking Syria
"International norms" refers to the near-universal ban on chemical weapons.
"America's core self-interest" refers to maintaining credibility by following through on the threat to strike Syria should the "red line" be crossed.
But there are fundamental problems with both of these arguments.
First of all, Syria is one of the few countries that has never signed the U.N. Chemicals Weapons Treaty. Not having signed the treaty, they are not legally bound to it. Although Syria has signed the Geneva Conventions and the Geneva Gas Protocols, which ban such things as violence against non-combatants and the use of chemical weapons, those conventions only apply to the use of weapons against foreign countries, saying nothing about their use on one's own people, as noted by Northwestern politics professor Ian Hurd.
Furthermore, the U.S. hardly has the high moral ground after it supported Iraq with full knowledge Saddam Huessein was gassing Iranians and Kurds in 1988. What possible argument would the U.S. have for supporting, not to mention not intervening in, Iraq after it used chemical weapons?
The ban against chemical weapons is of only limited use in the first place, as former intelligence analyst Joshua Foust explains:
The problem with the norm against chemical weapons use is that it goes out the window when things go wrong. Dictatorial regimes who already abuse their citizens will not be above further abuse when it comes to defending themselves from an existential threat. In Assad’s mind, he is fighting, literally, for his life?—?so any perceived outcry over using chemical weapons is going to be outweighed by his need for survival.
The only legal ways to intervene militarily in another country are in cases of self-defense or with U.N. Security Council approval. This isn't a case of self-defense, and getting through the U.N. won't be easy.
Phillip Carter, senior fellow and counsel at the Center for a New American Security, explains at War on the Rocks:
The first exception to the rule covers actions taken pursuant to a UN Security Council resolution. The US had this mandate in Libya, and for its recent actions in Afghanistan, Somalia, and the first Gulf War, among others. However, because Russia and China plan to veto any intervention in Syria that comes before the Security Council, there appears to be no chance of obtaining UN sanction to act in Syria.
After all, the "Syrian regime has not abdicated its sovereignty in a way that invites attack" any more than "the U.S. Civil War justified direct intervention by the British," Carter writes.
It's also tough to argue for a humanitarian intervention based on the death of 100,00 Syrians, when the U.S. mission is explicitly not to toppple the regime but only to send a message. In other words, it won't do much to stop the killing.
To put it simply, Washington proposes to violate international law in order to protect an international law which Syria has never officially promised to abide.
But just because it's illegal, doesn't mean that the war won't happen. After all, many claim that the 2003 U.S. invasion of Iraq was illegal, without a resolution from the UN Security Council. However, the US and UK argued that pre-existing resolutions from the Persian Gulf War in 1991 also authorized them to act in 2003, in order to defend Kuwait from immiment threat of Saddam's WMDs by "all necessary means."
Washington's intent in Syria, on the other hand, has no clearly defined legal justification — at least nothing resembling the Persian Gulf War — so Western lawyers have their work cut out for them.