急！why to say "British history has been a history of invasion"?

"British history has been a history of invasion". Please illustrate this point with the examples from the text. How did each of the invasions influence English culture ?
1. British history has been a history of invasions. Before the first century AD Britain was made up of many tribal kingdoms of Celtic people: a powerful culture originating in central Europe. Then in 43AD Britain was invaded by the Roman empire, and England and Wales (though not Scotland or Ireland) became a part of the Roman empire for nearly 400 years.Two more groups of invaders were to come after the English: from the late 8th century on, raiders from Scandinavia, the ferocious Vikings, threatened Britain's shores….