Complex sentence: The decline of Liverpool after the Second World War, and changing patterns of Welsh migration, caused the Welsh presence to diminish; in the 1960s the flooding of the Tryweryn Valley to provide the city with water soured relations with many people in Wales.
Split into simpler sentences: The decline of Liverpool after the Second World War and changing patterns of Welsh migration, caused the Welsh presence to diminish. In the 1960s, the flooding of the Tryweryn Valley to provide the city with water soured relations with many people in Wales.