Complex sentence: The RAF flew its first strategic bombing raid on Germany on 15 May 1940 (in the immediate aftermath of the German's bombing of Rotterdam) and in September 1940, the Luftwaffe began targeting British cities in' The Blitz'.
Split into simpler sentences: The RAF flew its first strategic bombing raid on Germany on 15 May 1940 (in the immediate aftermath of the Rotterdam Blitz). In September 1940, the Luftwaffe began targeting British cities in the'Blitz'.