Complex sentence: The Trafalgar Square Christmas tree has been an annual gift to the people of London by the city of Oslo each year since 1947 as a token of gratitude for British support of Norway during the Second World War.
Split into simpler sentences: The Trafalgar Square Christmas tree has been an annual gift to the people of London by the people of Norway each year since 1947. The tree is meant as a token of gratitude for British support of Norway during the Second World War.