Complex sentence: At the time of the Battle of Britain in 1941 20 mm cannon were only just starting to arm the Spitfire and Hurricane, but by 1943 the RAF had converted entirely to cannon armament for its fighters.
Split into simpler sentences: At the time of the Battle of Britain in 1941,20 mm cannon were only just starting to arm the Spitfire and Hurricane. By 1943, the RAF had converted entirely to cannon armament for its fighters.