Rajan Menon points out that the core administration argument for attacking Syria is hollow:

The president has also stated that it’s essential to ensure that the bans on chemical weapons are respected. Yet the 1925 Geneva Protocol contains no provisions for unilateral enforcement by states, let alone via military force. The same goes for the Chemical Weapons Convention (which Syria has not signed). It calls for “collective measures…in conformity with international law” to address serious breaches. There’s no basis for the United States to don the mantle of self-styled enforcer [bold mine-DL].

And the legal case for unilateral action is further weakened by the lack of a self-defense rationale under the terms of the UN Charter: Assad has not used chemical (or any other) weapons against the United States. French president Francois Hollande says that “international law must evolve with the times,” but if the evolution occurs because of unilateral moves that lack wider support, then we’re on a slippery slope. One day, the global balance of power may be different and the dominant power may want international law to evolve to suit its purposes and make the same argument.

It doesn’t make sense to violate a central international legal prohibition for the sake of upholding another one, especially when the kind of “enforcement” being proposed has no basis in the relevant agreements. It is even harder to justify said “enforcement” when there is little reason to think that the norm in question will be seriously jeopardized in its absence. As administration officials keep saying, almost the entire world has accepted the Chemical Weapons Convention and adheres to it, and it is not at all likely that the states that approved this convention are going to abandon their adherence to its provisions because the U.S. didn’t bomb the Syrian government. So when the administration claims that this is about more than the Syrian government’s behavior, it is greatly exaggerating to drive people towards its position. The U.S. is not obliged to take military action in this case, and it is specifically prohibited from doing so under the circumstances. Since the U.S. isn’t acting in self-defense, and the administration isn’t even trying to pretend that it is, there’s no good argument left for this attack. Bombing another country out of frustration with an intractable conflict is the opposite of a sound policy.

While there are some supporters of Obama’s proposed attack that defend it specifically on the grounds of enforcing international norms, it’s telling how many more feel compelled to resort to other arguments that have nothing to do with norm-enforcement. One reason for this may be that some of them assume that this is not a good enough reason by itself to justify military action, and another may be that they correctly understand it is not the sort of argument that will have broad appeal, and others want to treat the use of chemical weapons as the occasion to get the U.S. into a war that they’ve wanted it to join from the beginning. What supporters of this intervention should bear in mind is that they are backing the military action that Obama would actually be ordering for the reasons that he gives for ordering it. It will probably not end up being the kind of intervention they have in mind, nor will it be done for the sake of their preferred causes. No matter how hawkish or antiwar you are, if you don’t have confidence in the administration’s case as it has been stated publicly you should oppose the resolution before Congress.