Why care if Syria uses chemical weapons? This:
Let me explain why. If we fail to act, the Assad regime will see no reason to stop using chemical weapons. As the ban against these weapons erodes, other tyrants will have no reason to think twice about acquiring poison gas, and using them. Over time, our troops would again face the prospect of chemical warfare on the battlefield. And it could be easier for terrorist organizations to obtain these weapons, and to use them to attack civilians.
If fighting spills beyond Syria’s borders, these weapons could threaten allies like Turkey, Jordan, and Israel. And a failure to stand against the use of chemical weapons would weaken prohibitions against other weapons of mass destruction, and embolden Assad’s ally, Iran — which must decide whether to ignore international law by building a nuclear weapon, or to take a more peaceful path.
This is not a world we should accept. This is what’s at stake. And that is why, after careful deliberation, I determined that it is in the national security interests of the United States to respond to the Assad regime’s use of chemical weapons through a targeted military strike. The purpose of this strike would be to deter Assad from using chemical weapons, to degrade his regime’s ability to use them, and to make clear to the world that we will not tolerate their use.
That’s my judgment as Commander-in-Chief. But I’m also the President of the world’s oldest constitutional democracy. So even though I possess the authority to order military strikes, I believed it was right, in the absence of a direct or imminent threat to our security, to take this debate to Congress ….
Obama address to nation on Syria, 9-10-13