Original sentence: The bomb exploded, but when he turned his Lancaster to assess the damage it turned out that only a section of the crest of the dam had been blown off: the main body of the dam itself was still functional
Edited sentence: (It was just as well the Germans had thought this dam to be invulnerable to air attack: they had placed no anti-aircraft guns in this area.) The bomb exploded, but when he turned his Lancaster to assess the damage it turned out that only a section of the crest of the dam had been blown off: the main body of the dam itself was still functional