To clarify a lot of confusion I see in the comments. This book considers a country to be invaded when:

• At any point during the history of England, Whales, northern Ireland, Scotland, and or significant predecessors any army of theirs “invaded”,

• Counting any part of the land that is now considered part of the UN country. So not only from when the country existed,

• And being invaded meaning they had some control over, or fought conflicts in the territory.