German plans to invade Britain appeared as early as 1939, but it wouldn't be until the collapse of the French Third Republic that the idea took on an added fervor. In early July 1940, Hitler issued Directive No. 16 for "Preparations of a Landing Operation against England," a plan that would evolve into Operation Sea Lion.

The German plan was ambitious--if not completely unrealistic. After reviewing the directive, Grossadmiral Erich Raeder complained to Hitler about Germany's naval inferiority as it stood in contrast to the Royal Navy. He argued that the Kriegsmarine couldn't possibly provide sufficient transports for the 40 divisions which the Wehrmacht planned to land; the German Navy, with only a few weeks to prepare, were suddenly tasked with transporting the first wave of 100,000 men, along with tanks, motor transport, and equipment across the Channel. And it would have to do so against the formidable Royal Navy and Royal Air Force (RAF). Raeder explained to Hitler that an invasion should only be attempted after the German air force, known as the Luftwaffe, achieved air superiority over the Channel and along the English coast.

By comparison, the Normandy landings of 1944 involved 156,000 men from over a dozen countries spread across 10 divisions. But unlike the Allies, the German planners didn't have the benefit of an entire year to prepare, nor the industrial might of the United States.

German forces would have to cross the choppy waters of the 20 mile long (32 km long) English Channel, which was dubbed the "greatest anti-tank ditch in the world." What's more, the German army, which had a long and prestigious history of land warfare, had never attempted an amphibious landing quite like this. Likewise, its navy was of little help. German military planners weren't entirely sure how to proceed, and considered deploying canal barges across the Channel, debated whether or not to invade beaches or seaports, and mulled over the tremendous logistical challenges. 

Hitler himself became lukewarm about the invasion. He wondered about the geopolitical ramifications of a defeated British Empire, and was concerned that its undefended colonies would be gobbled up by Japan, Russia, and the United States. As the weeks went by, and as the unrealistic timelines became increasingly obvious, few in the German high command took Operation Sea Lion very seriously. As noted by historian Antony Beevor in The Second World War, the "invasion of Britain was never treated with urgency at the highest levels."