Context:

Explicitly quoted at the beginning of the episode, described as 'one of the most famous passages of historical prose ever written,' discussing Gibbon's view on the Battle of Tours and Charles Martel

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Referenced when discussing Hitler's views on the Battle of Tours - 'he spelt it out in his table talk' - followed by a direct quotation from the work

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Explicitly named as the source for Bede's account of comets and the Saracen invasion of Gaul

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Described as 'whose book on early Carolingian warfare is brilliant on this whole campaign' - quoted regarding Arab military technology and composite recurve bows

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Described as 'a chronicle that was written by an anonymous Christian priest back in Spain' and identified as 'essentially our main source, our most contemporary source for what's going on' regarding the Battle of Tours

Context:

Referenced for his depictions of early medieval combat - 'it's a bit like the images of fighting that you get in Bernard Cornwell's books' - describing shield wall fighting