Complex sentence: The French were unable to recover Rivière's body from the battlefield, but several weeks after the battle they heard rumours that it had been savagely mutilated and buried near the Black Flag stronghold of Phu Hoai.
Split into simpler sentences: The French were unable to recover Rivière's body, and for several months were unsure of the precise circumstances of his death. Several weeks after the battle the French heard rumours that Rivière's body had been savagely mutilated and buried near the Black Flag stronghold of Phu Hoai.