Complex sentence: Serong later became a counter-insurgency advisor to General Paul Harkins, Commander US MACV, but this relationship ended under Pollard's successor, Lieutenant General John Wilton, who reduced Serong's autonomy while continuing to grant him considerable latitude.
Split into simpler sentences: Serong later became a counter-insurgency advisor to General Paul Harkins, Commander US MACV. However, this relationship ended under Pollard's successor, Lieutenant General John Wilton, who acted to reduce Serong's autonomy even if he was continued to be allowed considerable latitude.