Complex sentence: Other Gaullists, those who could not join Britain (that is, the overwhelming majority of them), remained in the territories ruled by Vichy and supported by soviet funds and weapons built networks of propagandists, spies and saboteurs to harass and discomfit the occupiers.
Split into simpler sentences: Other Gaullists, those who could not join Britain (that is, the overwhelming majority of them), remained in the territories ruled by Vichy. The Allies helped build networks of propagandists, spies and saboteurs to harass and discomfit the occupiers.