The White House issued a cautious public statement about the attacks from a deputy spokesman shortly before noon, but behind the scenes the president and his national security team were grappling with the urgency and enormity of the event: the largest mass killing of the Syrian civil war, and most likely the deadliest chemical weapons attack since Saddam Hussein’s troops killed thousands of Kurds with sarin gas during the waning days of the Iran-Iraq war in 1988.