By now, it is clear the USA and other international actors are contemplating military strikes against the Syrian regime. Chances are that Russia will oppose this in the Security Council. If the USA and others still launch strikes they will be technically illegal under international law.
What were are seeing is the collision of two powerful international norms, which apparently cannot coexist when it comes to international policy on Syria.
The first is that there are a set of weapons so horrific, so indiscriminate by their very nature that their use must be totally eliminated from modern warfare. Chemical weapons fall into this category, as might land mines and nuclear weapons. But chemical weapons are unique in that the norm has been in place since the end of World War One. It is very deeply established–much more than the Land Mine Convention. To be sure, there have been violations, like the Iran-Iraq war of the 1980s. But for the most part the taboo against using chemical weapons in warfare has held strong.
The long term goal of this norm is to make war somewhat less brutal, and somewhat less indiscriminate. Fewer civilians have died in wars because warring parties have refused to stock chemical arsenals.
The second norm is the prohibition against one country invading, bombing or otherwise intervening militarily in another country without the expressed consent of the Security Council. Under the UN Charter every country is granted the inherent right of self-defense, but it is only the Security Council that makes international intervention legal. This is a very clear legal principle set out in the UN Charter that adds legitimacy to interventions that the international community agrees upon (like Mali). It also puts breaks on war, by imposing a set of rules on who gets to declare war and in what circumstances.
There may be good reasons to violate that law, like in Kosovo, but that violation comes at a high cost. You can draw a direct line from the Clinton administration’s technically illegal (but perhaps morally justified) intervention in Kosovo to the Bush administration’s illegal invasion of Iraq. When you chip away at the norm that only the Security Council holds the power to authorize an international intervention you make international interventions more likely, whether for good or ill.
What we are seeing in Syria today is a collision of those two norms. The question is, which would you prefer be violated: That chemical weapons use be punished or deterred; or the international legal precept that countries cannot go to war with another unless the Security Council gives the green light?
This is the key question with which those of us who believe in the value of a strong international system and international law must wrestle. There is no clear answer, and there are high costs to either choice.