International Law, like the British constitution, does not exist in one canon of documents but is established through a mind-blowing network of treaties, agreements and precedents and has no single authority to enforce it. There simply will be no conclusive judgement on the matter, as there wasn't with the legality issue on the Iraq invasion. Might can be right and so can the force of international diplomatic and economic pressure. It seems that de jure authority will always play second fiddle to de facto power. Israel may either prosper or suffer as a result but the legions of lawyers and professors currently working on Tuesday morning's events won't be the ones who decide the outcome.
This is probably the most precise summary of the situation that I've heard so far.
People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.