Credibility

That's all that this is really about. The administration feels that it is boxed in because:

1. Obama called the use of chemical weapons in Syria a "red line" that cannot be crossed without "changing [the President's] calculus' which was widely interpreted as a threat to use military force.
2. The administration has now publicly stated that the chemical weapon attack last week was perpetrated by the Assad regime.

That's it, that's the entire reason that American weapons will kill Syrians in the near future. The administration has made some statements that seem to logically require an attack and there's no way to not kill someone without looking bad.

It's not about actually stopping any chemical weapons use (except maybe under a vague deterrence theory that if the U.S. does not act, Assad and others will feel empowered to use chemical weapons in the future. Never mind that an ineffective attack, which is what we are probably looking at, would too.).

It's not about "protecting civilians" because  bombing a foreign country will result in more civilian deaths, not less.

It's not even about overthrowing Assad.

It's about killing people to avoid embarrassment.


ADDING: what Atrios just posted.