In the early morning of 9 April 1940 Germany occupied Denmark and invaded Norway. The evening before, German bombers had launched their biggest attack so far that spring on the British fleet in Scapa Flow. On the night of 10 April they came back, around 60 aircraft, to face a barrage of anti-aircraft fire from the batteries below. What was happening in the air that spring? How close was it? Has its significance been fully recognised? Four aviation historians discuss – Commander David Hobbs, Ian Hutchison, Anthony J. Cumming and Paul Beaver.