In 2011 Barack Obama led an allied military intervention in Libya without consulting the US Congress. Last August, after the sarin attack on the Damascus suburb of Ghouta, he was ready to launch an allied air strike, this time to punish the Syrian government for allegedly crossing the ‘red line’ he had set in 2012 on the use of chemical weapons.＊ Then with less than two days to go before the planned strike, he announced that he would seek congressional approval for the intervention. The strike was postponed as Congress prepared for hearings, and subsequently cancelled when Obama accepted Assad’s offer to relinquish his chemical arsenal in a deal brokered by Russia. Why did Obama delay and then relent on Syria when he was not shy about rushing into Libya? The answer lies in a clash between those in the administration who were committed to enforcing the red line, and military leaders who thought that going to war was both unjustified and potentially disastrous.