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...