Complex sentence: As the circuits and underground movements in occupied Europe grew, the needs of the SOE grew and the air units supporting them were expanded, reaching a peak effort just around the time of the Allied invasion of occupied Europe in June 1944.
Split into simpler sentences: As the circuits and underground movements in occupied Europe grew, the needs of the SOE grew and the air units supporting them were expanded. The peak effort was reached just around the time of the Allied invasion of occupied Europe in June 1944.