Complex sentence: Although constructed as an all-weather night fighter airfield for the defence of Liverpool, it did not open until 7 December 1941 which was just after the Liverpool Blitz, which had peaked in May.
Split into simpler sentences: Although constructed as an all-weather night fighter airfield for the defence of Liverpool, it did not open until 7 December 1941. This was just after the Liverpool Blitz, which had peaked in May.