Secretary of State John Kerry on Syria a few minutes ago:
"It matters because if we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity, even after the United States and our allies said no, and then the world does nothing about it, there will be no end to the test of our resolve and the dangers that will flow from those others who believe that they can do as they will.
It is also profoundly about who we are. We are the United States of America. We are the country that has tried, not always successfully, but always tried to honor a set of universal values around which we have organized our lives and our aspirations."
The United States hasn't always lived up to the values it professes, but I think it's important to invoke them and act on them in situations like this.