Actually, he did.

Via TIME:

TIME: Can you get a majority of your caucus? Is that important?

PELOSI: I don’t know. I think it would be important to get a majority in the Congress. But I don’t know if it’s important how you would break it down. These issues are not really partisan.

Are we now going to be doing this every place? No. It is in our national security interest to do it, because [Assad] crossed a line when he used chemical weapons. President Obama did not draw a red line. That was drawn by humanity decades ago, to say that this is unacceptable. To ignore it is only to encourage those who would follow suit.

That is, I think, the challenge that we all have, that the president certainly has: [to present] the evidence that he did this, why is it in our interest to do something about it, what does it mean to regional stability, to our national security and to global security. And in think there is a very strong case that can be made in those regards.