According to an interview with Julian Fellowes, creator and writer of Downton Abbey, the theme of the series is about people confronting change whether they like it or not. The show started in 1912, just before the Great War, but in the 1920’s there was an accelerated rate of change. It was actually the end of the Second World War that was the coup de grace for
Crawley type people.
Jessica Brown Findlay who played Lady Sybil, and Dan Stevens who played Mathew Crawley wanted to leave after season three, so they had to be killed off—Sybil in childbirth and Mathew in a car crash.
The back story about Robert and Cora is how she came to
as an American heiress and
met Robert who married her for her money and then fell in love. England