。
They controlled one-fifth of the territory.
Literal
They [topic-は] territory [of-の] 5-[of-の]-1 [object-を] controlled.
~分の~ is the Japanese fraction format: [denominator]分の[numerator]. 5分の1 ('one-fifth') — literally 'one of the five parts.' Japanese says the denominator first, opposite to English's numerator-first. 支配する ('to control, rule over') is formal.