Frontline: In August of 2013, a rebel-held suburb of Damascus was attacked with sarin gas — a nerve agent that causes lung muscle paralysis and results in death from suffocation.

The attack killed 1,400 men, women and children, and at the White House, officials asserted “with high confidence” that the government of Bashar al-Assad was responsible.

One year earlier, President Barack Obama had described Assad’s potential use of chemical weapons as “a red line” that would have “enormous consequences” and “change my calculus” on American military intervention in Syria’s civil war.

When Assad appeared to cross that line, Obama ordered the Pentagon to prepare to attack.

“Our finger was on the trigger,” Gen. Martin Dempsey, Chairman of the Joint Chiefs of Staff, tells veteran FRONTLINE correspondent Martin Smith in Tuesday’s new documentary, Obama at War. “We had everything in place and we were just waiting for instructions to proceed.”

But as FRONTLINE details in the below excerpt from Obama at War, the president had second thoughts. [Continue reading…]