Kerry & Manning are depicted as being in agreement the slaughter of innocents is a moral obsenity.
Manning took what action he could in the hopes it would deter further atrocities.
The Obama administration wants to take action it deems necessary to deter further atrocities of chemical attacks on innocent civilians.
Obama may very well be mistaken that punitive strikes against Assad's military assets will accomplish that goal, but then again it might.
Personally, I oppose military action in Syria. For one thing, there appears to be more to this than a red line drawn against chemical weapons. We have taken other measures to aid the rebels, getting involved in a civil war. There's also been rationalizations that we need to protect Israel and send a message to Iran -- neither of which justify attacking the Assad regime in Syria. Most of all, I oppose this action because I think it will do very little if any good, and the downside of unintended consequences is potentially high.
All of that, plus it may very well be a violation of international law without UN authorization. I realize there is no chance of a UN resolution with Russia having a veto in the Security Council, so let's launch a major diplomatic initiative to turn this against Russia's obstructionism and support of a dictator who gasses his own people.
A military action for humanitarian purposes must be within the scope of international law, or a clear-cut case of justifiable self-defense. Obama's proposed action in Syria meets neither of those criteria.