Complex sentence: The Decline of the Byzantine Empire was in fact similar to the decline of the western roman empire, a process lasting many centuries, the is no consensus on when this process began but many dates and time lines have been proposed by historians.
Split into simpler sentences: The decline of the Byzantine Empire was a process similar to the decline of the Western Roman Empire, in that it lasted many centuries. There is no clear consensus on when this process began; but many dates and time lines have been proposed by historians.