9/11, 20 years on: 'A milestone in global history'

As the US prepares to mark 20 years since the 9/11 attacks that transfixed and horrified the world, we speak to terrorism expert Cyrille Bret, author of "Ten Attacks That Changed the World". He argues that the events of that terrible day not only transformed our collective experience of safety, but also reignited the debate over how much freedom we're prepared to relinquish in the name of security.

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