Complex sentence: The entire northwestern-to-eastern arc of the kingdom was surrounded by the Shan states, who unlike the Mongols, did not leave and would become major players in Burmese (and South-East Asian) history.
Split into simpler sentences: The entire northwestern-to-eastern arc of the kingdom was surrounded by Shan States. The Shans, unlike the Mongols, did not leave and would become major players in Burmese (and South-East Asian) history.