Complex sentence: The first German air attacks on Harstad began on 16 April, but anti-aircraft defences prevented serious damage until a raid on 20 May destroyed oil tanks and civilian houses and another raid on 23 May hit Allied shipping in the harbour.
Split into simpler sentences: The first German air attacks on Harstad began on 16 April, but anti-aircraft defences prevented serious damage. Only in late May did the Germans succeed in causing destruction in Harstad; a raid on 20 May destroying oil tanks and civilian houses and another raid on 23 May hitting Allied shipping in the harbour.