Survivors recount the horrors they endured on 9/11. Immediately after the attacks, George W. Bush begins charting America's aggressive military response.

Learning the hijackers benefitted from gaps in US intelligence, the White House justifies a secret surveillance program and "enhanced interrogation."

With no cohesive strategy, the war in Afghanistan squanders early successes and backfires, fueling disillusionment, corruption and a Taliban resurgence.

After two decades, the US aims to end its longest-ever war, but its withdrawal threatens to plunge Afghanistan back into repressive, bloody violence.