During the summer of 1940, Hitler's Germany appeared unstoppable. The Nazis were masters of mainland Europe, in alliance with Stalin's Russia, and only the English Channel prevented an immediate invasion.
Britain stood alone. The BEF had been routed but, due to the "Dunkirk miracle," most of her manpower had returned, albeit without their transport and heavy equipment and guns. There was no doubt that the Nazis planned to invade—all intelligence pointed that way. However, it never materialized, thanks to the outcome of the Battle of Britain and Hitler's decision to invade Russia.
Operation Sea Lion examines just how realistic the German threat of invasion was. The author studies the plans, the available capability and resources, and the Germans' record in Norway and, later, Crete. The author weighs these against the state of Britain's defenses and the relative strengths of the land, air, and naval forces.
The result is a fascinating study of what might or might not have been.