President Obama said the situation in Syria represented a "grave threat" to "global peace and security." He said failing to attack Syria in the wake of its use of chemical weapons would lead to unraveling of international norms against the use of chemical weapons. "The question is: 'Do these norms mean anything?'," he asked.
President Obama also said that he discussed the situation in Syria with Russian president Vladimir Putin. He said that while Putin and he disagreed on the question of how to respond to the use of chemical weapons, they both agreed that in the long-term the only way to solve the Syrian civil war is through political and diplomatic means.
The president also addressed one of the biggest questions opponents of the attack have: What would happen if the U.S. attacked Syria, but Assad nonetheless used chemical weapons again. "Is it possible that Assad doubles down in the face of our action and uses chemical weapons more widely? I suppose anything is possible. But it wouldn't be wise. I suppose at that point it would be easier to mobilize the international community, not harder. [...] And we would gladly join an international coalition to make sure that it stops."
In other words, if Assad were to double down, things would escalate—and President Obama would favor that escalation.
On the question of what would happen if one or both chambers did not give him authorization, Obama didn't take a definitive position, but said he went to Congress because "I could not honestly claim that the threat posed by Assad's use of chemical weapons ... posed an imminent threat to the United States." That's not to say he doesn't think it warrants the use of U.S. military force (obviously, he does), but given the lack of an imminent threat, he didn't believe unilateral executive action was necessary or warranted.
As of 10:25 AM ET, the press conference was still ongoing. This post will be updated as needed.
7:31 AM PT: President Obama didn't directly answer the question of whether he would attack even without congressional authorization. "What I have said, I will repeat. I put this before Congress because I think we will be in a stronger position" if Congress authorizes force, he said.
"It's conceivable that at the end of the day, I don't persuade the American people that this is the right thing to do." But he also said that "these" sorts of attacks are always unpopular, suggesting that he would proceed. "The intervention in Kosovo was very unpopular," he said. (The House failed to authorize force there.) "But it was the right thing to do."
7:34 AM PT: While the president was explicitly ambiguous on whether he will heed Congress's decision, he nonetheless distanced himself from earlier comments by deputy National Security Adviser Tony Blinken who said the president has no "intention" of attacking Syria without congressional authorization.
7:36 AM PT: On proposals like the Manchin-Heitkamp proposal to give Assad a 45-day window to renounce the use of chemical weapons, the president said he was listening, but: "So far, at least, I have not seen proposals presented that I think, as a practical matter, would do the job."