"That wasn't something I made up," he said. "I didn't pluck it out of thin air. There's a reason for it."
Obama said that if the world fails to act, "we give lip service to the notion that these international norms are important."
And that, he said, would embolden despots and repressive regimes around the world to flout all sorts of international standards.
"The moral thing to do is not to stand by and do nothing," he declared at a news conference in Stockholm with Swedish Prime Minister Fredrik Reinfeldt.
Back home, the Senate Foreign Relations Committee could vote on authorizing the use of force as early as Wednesday, the first in a series of votes as the president's request makes its way through Senate and House committees before coming before the two chambers for a final vote.
And in a setback to Obama's push for backing on Capitol Hill, Sen. John McCain said he doesn't support the Senate resolution.
McCain has been an outspoken advocate of intervention and wants more than cruise missile strikes and other limited action, although he has said he doesn't favor U.S. combat troops on the ground there.
Sending a message to Congress from afar, Obama said Wednesday there was far more than his own credibility at stake.
"I didn't set a red line, the world set a red line," he said. "The world set a red line when governments representing 98 percent of world population said the use of chemical weapons are abhorrent." He added that Congress set a red line when it ratified the treaty.
With Obama in Europe, his top national security aides were to participate in a series of public and private hearings at the Capitol Wednesday to advance their case for limited strikes against Syrian President Bashar Assad's regime in retaliation for what the administration says was a deadly sarin gas attack by his forces outside Damascus last month.
The Senate Foreign Relations Committee's top members drafted a resolution late Tuesday that permits Obama to order a "limited and tailored" military mission against Syria, as long as it doesn't exceed 90 days and involves no American troops on the ground for combat operations.
"We have pursued a course of action that gives the president the authority he needs to deploy force in response to the Assad regime's criminal use of chemical weapons against the Syrian people, while assuring that the authorization is narrow and focused," said the committee's chairman, Sen. Bob Menendez, D-N.J., who drafted the measure with Sen. Bob Corker of Tennessee, the panel's senior Republican.
"We have an obligation to act, not witness and watch while a humanitarian tragedy is unfolding in plain view," Menendez said.
Obama also needed to persuade a Republican-dominated House that has opposed almost the entirety of Obama's agenda since seizing the majority more than three years ago. Several conservative Republicans and some anti-war Democrats already have come out in opposition to Obama's plans, even as Republican and Democratic House leaders gave their support to the president Tuesday.
House Speaker Boehner emerged from a meeting at the White House and declared that the U.S. has "enemies around the world that need to understand that we're not going to tolerate this type of behavior. We also have allies around the world and allies in the region who also need to know that America will be there and stand up when it's necessary."
Rep. Eric Cantor, the House majority leader, also backed action. But he acknowledged the split positions among both parties and said it was up to Obama to "make the case to Congress and to the American people that this is the right course of action."