A factual work of military history based on years of research by the author. Operation Sea Lion was the German code name for the invasion of Great Britain in the autumn of 1940. This work focuses on the actual ability of the British armed forces to repel such an invasion. It shows the naval, army and air forces locations, equipment states, organisation, numbers and movements at that time. It shows how the political and military leadership prepared for the vital task of defending this nation and, whilst it does not prophesise as to the outcome, it looks at how each of the potential stages of invasion could have developed with the information available from all sources.