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Twenty years ago, on 11 September 2001, Al-Qaeda began four coordinated terrorist attacks on the US, lasting one hour and seventeen minutes.

The world watched as nineteen terrorists crashed four planes - two into the World Trade Centre, one into the Pentagon, the very symbol of American might, and the fourth into a field in Pennsylvania.

To this day, Al-Qaeda’s attack 9/11 remains the deadliest terror attack in history.

It was the audacity of the attack that was so shocking. The idea that in a little over an hour the United States of America - the leader of the free world - could be shown to be utterly vulnerable, not invincible.

That terrible day arguably has impacted every American psyche to this day, the way America sees its place in the world and the way we see America.

Newsnight’s David Grossman reports on how September 11th changed the world and how we can counter the modern face of terrorism.