Complex sentence: The attack on 28 March 1942 created a firestorm, that caused severe damage to the historic centre and the Bombing of Lübeck in World War II destroyed three of the main churches and greater parts of the built-up area.
Split into simpler sentences: The attack on 28 March 1942 created a firestorm that caused severe damage to the historic centre. The Bombing of Lübeck in World War II destroyed three of the main churches and greater parts of the built-up area.