Some human rights groups have long pushed for international law to allow outside intervention to stop atrocities. The United States and other nations have been reluctant to accept such a broad change, Bellinger said.
The administration's lawyers have been careful to guide the choice of words used by top officials. Obama and Secretary of State John Kerry, who made a forceful case for military action on Friday, have carefully portrayed al Assad's actions as violating "international norms."
That's in part because Syria isn't among the 188 countries, including the United States, that signed the Chemical Weapons Convention, the treaty that prohibits the production and use of such weapons.
Obama has long grappled with the issue of how to deal with civil war atrocities when international law doesn't offer a way to stop them. In his December 2009 speech to accept the Nobel Peace Prize, Obama endorsed a call for an "evolution of human institutions."
On one hand, he said, it is important for the United States and other nations to respect international rules, saying "when we don't, our actions appear arbitrary and undercut the legitimacy of future interventions, no matter how justified."
But then he also noted the need to "prevent the slaughter of civilians by their own government, or to stop a civil war whose violence and suffering can engulf an entire region."
Obama added: "I believe that force can be justified on humanitarian grounds, as it was in the Balkans, or in other places that have been scarred by war. Inaction tears at our conscience and can lead to more costly intervention later. That's why all responsible nations must embrace the role that militaries with a clear mandate can play to keep the peace."