One of the greatest Muslim empires was that of the Mughals who ruled from the city of Delhi in northern India. The empire came to an end 150 years ago. William Dalrymple is the author of a new book called 'The Last Mughal'.
Here he describes how he found 20,000 largely unstudied documents in the national archive of India.
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Dalrymple also researched 'White Mughals' - Westerners who took the ways of Indians during the early 19th century. One was David Ochterlony, Boston-born and loyal to the British during the American Revolution.
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The book focuses on the story of Zafar, the last of the Mughal emperors.
Marco Werman talked with Dalrymple