Why didn't Darius and Xerxes do history? [audio 29min]

In the late 6th century BC, Darius was the most powerful man on the planet. King of Persia, he also ruled the largest empire that the world had ever seen. His son Xerxes led one of the most famous military victories at Thermopylae. But little is recorded of these huge events. So why didn't the Persians do history? Presenter Tom Holland goes in search of Darius and his version of history.