Complex sentence: In the "Blitz", German bombers were dispatched individually; alternately each "Gruppe" had been briefed to take off and bomb over a period of time extending anywhere up to ten hours during the darkest period.
Split into simpler sentences: In the "Blitz", German bombers were dispatched individually rather than in formation. Each "Gruppe" had been briefed to take off and bomb the target over a period of time extending anywhere up to ten hours during the darkest period.