In Volume Three, Gibbon picks up his story with the chaotic events following the retreat of the Eastern Roman Empire from the Italian peninsula in the seventh century. As the German conquerors settle into the new territories they took from the old Roman Empire, Islam suddenly emerges on the Arabian peninsula and sweeps across North Africa and into Spain. Muslim forces move north and threaten Constantinople itself.

This marks the beginning of an 800 year struggle between the Byzantine Empire and Islam. Gibbon describes in breathtaking style the last years of Constantinople and its final death struggle with the Ottoman Empire.