Original sentence: By the time of his death, he had conquered the entire Achaemenid Persian Empire, adding it to Macedon's European territories; according to some modern writers, this was much of the world then known to the ancient Greeks (the 'Ecumene')
Edited sentence: By the time of his death, he had conquered the entire Achaemenid Persian Empire, adding it to Macedon's European territories; according to some modern writers, this was most of the world then known to the ancient Greeks (the 'Ecumene')