Complex sentence: Buddhism in Mongolia began with the Yuan emperors conversion to Tibetan Buddhism; however, the Mongols returned to their old shamanist ways after the collapse of their empire and it wasn't until the 16th and 17th centuries that Buddhism reemerged.
Split into simpler sentences: Buddhism in Mongolia began with the Yuan emperors conversion to Tibetan Buddhism. The Mongols also returned to their old shamanist ways after the collapse of their empire and it wasn't until the 16th and 17th centuries that Buddhism reemerged.