Apparently, there was a period of time (Wikipedia says 618-842 CE) in which the Tibetan Plateau and a significant portion of land outside it was united under a single empire. After a civil war, the plateau was never again unified. What kind of mark or significance did the Tibetan Empire have on human history? Would anything have changed should it persisted past its end in some form or other?
Followup question: How effective, if at all, was this empire at spreading the religion of Buddhism?
Side question: To what extent is the term 'medieval' applicable to East Asian history.