The use of chemical weapons in Syria is a potential game changer regarding the war there and how the United States responds to it.
The White House announced that it now has credible evidence that the Assad regime has used chemical weapons against opposition rebels.
President Obama has said that the regime's use of chemical weapons would cross a "red line" and change his thinking.
Thursday afternoon a senior administration official told reporters that that has now happened.
But the official also said that the President will decide what the US response will be according to his own timeline.
James Traub is a columnist for Foreign Policy.