Defense Secretary Chuck Hagel raised the possibility that Assad's chemical weapons stockpile, considered one of the world's largest, could be seized by his allies, including the Lebanon-based militant group Hezbollah. "We cannot afford for Hezbollah or any terrorist groups determined to strike the United States to have incentives to acquire or use these chemical weapons," Hagel told the House Foreign Affairs Committee.
Vali Nasr, a former senior official in Obama's State Department, said Syria's spiraling death toll, the rise of fighters in Syria associated with al-Qaida and other extremist groups, and pressure on neighboring nations from a flood of refugees have already threatened U.S. security interests for years.
"For a very long time we reduced Syria to just a humanitarian tragedy that, as bad as it was, was not a sufficient cause for American involvement," said Nasr, now dean of the Johns Hopkins School of Advanced International Studies. "That meant we ignored all the other ways in which Syria was a national security threat. And for two years we tried to minimize the impact of Syria, and now all of a sudden the administration finds itself in the position of having to give sufficient urgency to Syria to justify action."
Over the past two years, the White House has mightily resisted intervening in Syria's civil war with U.S. military force. A year ago, Obama signaled the one "red line" exception would be the use of chemical weapons.
At the same time, the U.S. has used a heavy hand in years of negotiations with Iran as world powers try to persuade Tehran to significantly scale back its nuclear program, and seek to prevent its ability to build a bomb.
And Washington has repeatedly and sternly warned North Korea against launching underground nuclear tests and missiles that have rattled its regional neighbors and raised concerns that Pyongyang is building a nuclear-tipped rocket that can reach the United States.
"Iran and North Korea are carefully watching our next move," Rep. Ileana Ros-Lehtinen, R-Fla., said during the House hearing Wednesday. "A refusal to act in Syria after the president has set such a clear red line will be seen as a green light by the Iranian regime, who will see that we don't have the will to back up our words."
The administration's credibility was already at risk, however, after its muted response to a series of small-scale chemical weapons attacks this spring in Syria that killed a few dozen people.
As a result of those attacks, Obama pledged in June to increase aid to certain vetted rebel groups fighting Assad in a package that officials said included some weapons. But the aid did not start flowing until very recently and, overall, fell far short of being seen as a decisive or forceful action to punish Assad for the attacks.
Kerry on Wednesday said the scope of the August attacks — and strong intelligence indicating that Assad's government was to blame — convinced Obama that his red line had been crossed. Before now, "the president didn't want to rush into something," Kerry said.