Complex sentence: Definitions of ethnicity and nationality in Spain are politically fraught, particularly since the transition from Francoist Spain to the Kingdom of Spain in the 1970s, when regionalism became a major part of national politics.
Split into simpler sentences: Definitions of ethnicity and nationality in Spain are politically fraught. After the transition from Francoist Spain to the Kingdom of Spain in the 1970s, regionalism became a major part of national politics.