Complex sentence: Shimazu planned to retire soon in order to let the younger generation take over, however, upon hearing about the escalating conflict with Oda Nobunaga, he takes up his sword again and joins the fight to try to pave the way for the younger generation.
Split into simpler sentences: Shimazu planned to retire soon in order to let the younger generation take over. However, upon hearing about the escalating conflict between Oda Nobunaga, he takes his sword again and joins the fight to try to pave the way for the younger generation.