Barack Obama did not tell the whole story this autumn when he tried to make the case that Bashar al-Assad was responsible for the chemical weapons attack near Damascus on 21 August. In some instances, he omitted important intelligence, and in others he presented assumptions as facts. Most significant, he failed to acknowledge something known to the US intelligence community: that the Syrian army is not the only party in the country’s civil war with access to sarin, the nerve agent that a UN study concluded – without assessing responsibility – had been used in the rocket attack. In the...