Complex sentence: Its mission was to encourage and facilitate espionage, sabotage and reconnaissance behind enemy lines and in its early days, to serve as the core of the Auxiliary Units, a British resistance movement which would act in case of a German invasion of Britain.
Split into simpler sentences: Its mission was to encourage and facilitate espionage, sabotage and reconnaissance behind enemy lines. In its early days, it was also involved in the formation of the Auxiliary Units, a British resistance movement which would act in case of a German invasion of Britain.