The core legal obligation of the UN Charter requires member states to refrain from any use of force unless it can be justified as self-defence after a cross-border armed attack or mandated by a decision of the UN Security Council.
Neither of these conditions authorising a legal use of force is remotely present, and yet the discussion proceeds in the media and Washington circles as if the only questions worth discussing pertain to feasibility, costs, risks, and a possible backlash in the Arab world.
The imperial mentality is not inclined to discuss the question of legality, much less show behavioural respect for the constraints embedded in international law.
I'm not really convinced by the national sovereignty-at-all-costs arguments. Those allow a government to get away with anything in relation to those it governs up to and including genocide. If international laws and conventions are good for anything, they ought to be good for this. That's far from enough in itself to make any proposed humanitarian intervention a good idea but I don't think we do a good thing by ruling the idea out entirely based on national sovereignty.
A much more convincing argument is that it usually doesn't work.