The Sassanian Empire was the last in a series of Persian empires which had for centuries been seen as the rivals of Graeco-Roman civilization. It was also the last before the coming of Islam changed the entire character of Persian or Iranian civilization. In many ways the Sassanian Empire already had one foot in this new medieval world, yet its self-image was firmly cast in the mould of ancient Iran and the Empire regarded itself as the true heir of that great Achaemenid Persian Empire destroyed by Alexander the Great hundreds of years earlier.