I'm not a lawyer and I do not believe that legal opinions should always constrain state actions. But I do believe that widely shared international principles governing the behavior of states should be taken into consideration.
I await with interest how the Obama administration explains how its actions in Syria in response to apparent chemical weapons attacks -- whatever they may be -- fit within international law.
I think there are widely shared international principles -- the just war doctrine. When the Security Council was unable to act in Kosovo in 1999. NATO members decided to act. In my view, that constituted the "legitimate authority" required for action since it was a determination by an existing international organization that even operated under a unanimity principle.
Thus it would help if some other existing international organization formally authorized a punitive response to the use of the chemical weapons. I know U.S. military doctrine works to limit collateral damage and civilian casualties, another just war principle. Perhaps harder to justify is the principle that the actions "must have a reasonable chance of success." Although the goal of many nations is to overthrow the Assad regime, that is beyond the reach of limited air strikes, so some other goal must be chosen, for which there is a chance of success.
Let's see what the lawyers say, and whether they blush as they say it.