Complex sentence: The Viet Minh at this time controlled much of the border areas on northern Vietnam in Cao Bang, Bac Kan, and Lang Son, provinces in which the majority of the people were ethnic minorities and where the French had never gained full control.
Split into simpler sentences: The Viet Minh at this time controlled much of the border areas on northern Vietnam in Cao Bang, Bac Kan, and Lang Sonprovinces. The majority of the people in these provinces were ethnic minorities over whom the French had never gained full control.