Answer:
The answer is "Navy". Britain's powerful navy played a crucial role in protecting the country from French invasion during different periods in history, including during the Napoleonic Wars in the late 18th and early 19th centuries. The Royal Navy was one of the largest and most experienced naval forces in the world at the time, with a fleet of warships and skilled sailors. The British navy was able to control the seas and prevent the French from crossing the English Channel, thereby protecting Britain from invasion. The navy's role in defending Britain's shores and maintaining its global empire has been crucial throughout British history.