Complex sentence: Though not a British national, in February 1941 Atkins joined the French section of the SOE, originally in a secretarial capacity, but soon as assistant to section head Colonel Maurice Buckmaster and a de facto intelligence officer.
Split into simpler sentences: Though not a British national, in February 1941 Atkins joined the French section of the SOE as a secretary. She soon was made assistant to section head Colonel Maurice Buckmaster, and became a de facto intelligence officer.