Operation Sea Lion - Conflict & Date:
Operation Sea Lion was slated to take place sometime in late 1940, during World War II (1939-1945).
Operation Sea Lion - Background:
In the summer of 1940, shortly after Germany's stunning conquest of France, Adolf Hitler turned his attention to the final defeat of Britain. Somewhat surprised that London had rebuffed peace overtures, he issued Directive No. 16 on July 16 which stated, "As England, in spite of the hopelessness of her military position, has so far shown herself unwilling to come to any compromise, I have decided to begin to prepare for, and if necessary to carry out, an invasion of England...and if necessary the island will be occupied."
For this to succeed, Hitler laid out four conditions that had to be met to ensure success. Similar to those identified by German military planners in late 1939, they included elimination of the Royal Air Force to ensure air superiority, clearing of the English Channel of mines and the laying of German mines, the emplacing of artillery along the English Channel, and preventing the Royal Navy from interfering with the landings.
Though pushed by Hitler, neither Grand Admiral Erich Raeder of the Kriegsmarine or Reichsmarschall Hermann Göring of the Luftwaffe were avid supporters of the invasion plan.
Operation Sea Lion - German Planning:
Dubbed Operation Sea Lion, planning moved forward under the guidance of Chief of the General Staff General Fritz Halder. Though Hitler had originally desired to invade on August 16, it was soon realized that this date was unrealistic. Meeting with planners on July 31, Hitler was informed that most desired to postpone the operation until May 1941. As this would remove the political threat of the operation, Hitler refused this request but agreed to push Sea Lion back until September 16. In the early stages, the invasion plan for Sea Lion called for landings on a 200-mile front from Lyme Regis east to Ramsgate.
This would have seen Field Marshal Wilhelm Ritter von Leeb's Army Group C cross from Cherbourg and land at Lyme Regis while Field Marshal Gerd von Rundstedt's Army Group A sailed from Le Havre and the Calais area to land the southeast. Possessing a small and depleted surface fleet, Raeder opposed this broad front approach as he felt it could not be defended from the Royal Navy. As Göring began intense attacks against the RAF in August, which developed into the Battle of Britain, Halder vehemently attacked his naval counterpart feeling that a narrow invasion front would lead to heavy casualties.
Operation Sea Lion - The Plan Changes:
Bowing to Raeder's arguments, Hitler agreed to narrow the scope of the invasion on August 13 with the westernmost landings to be made at Worthing. As such, only Army Group A would take part in the initial landings. Composed of the 9th and 16th Armies, von Rundstedt's command would cross the Channel and establish a front from the Thames Estuary to Portsmouth. Pausing, they would build up their forces before conducting a pincer attack against London. This taken, German forces would advance north to around the 52nd parallel. Hitler assumed that Britain would surrender by the time his troops reached this line.
As the invasion plan continued to be in flux, Raeder was plagued by a lack of purpose-built landing craft. To remedy this situation, the Kriegsmarine gathered around 2,400 barges from around Europe. Though a large number, they were still insufficient for the invasion and could only be used in relatively calm seas. As these were gathered in the Channel ports, Raeder continued to be concerned that his naval forces would be insufficient to combat the Royal Navy's Home Fleet. To further support the invasion, a myriad of heavy guns were emplaced along the Straits of Dover.
Operation Sea Lion - British Preparations:
Aware of German invasion preparations, the British began defensive planning. Though a large number of men were available, much of the British Army's heavy equipment had been lost during the Dunkirk Evacuation. Appointed Commander-in-Chief, Home Forces in late May, General Sir Edmund Ironside was tasked with overseeing the island's defense. Lacking sufficient mobile forces, he elected to construct a system of static defensive lines around southern Britain which were backed by the heavier General Headquarters Anti-tank Line. These lines were to be supported by a small mobile reserve.
Operation Sea Lion - Delayed and Cancelled:
On September 3, with British Spitfires and Hurricanes still controlling the skies over southern Britain, Sea Lion was again postponed, first to September 21 and then, eleven days later, to September 27. On September 15, Göring launched massive raids against Britain in an attempt to crush Air Chief Marshal Hugh Dowding's Fighter Command. Defeated, the Luftwaffe took heavy losses. Summoning Göring and von Rundstedt on September 17, Hitler indefinitely postponed Operation Sea Lion citing the Luftwaffe's failure to obtain air superiority and a general lack of coordination between the branches of the German military.
Turning his attention eastward to the Soviet Union and planning for Operation Barbarossa, Hitler never returned to the invasion of Britain and the invasion barges were ultimately dispersed. In the years after the war, many officers and historians have debated whether Operation Sea Lion could have succeeded. Most have concluded that it likely would have failed due to the strength of the Royal Navy and the Kriegsmarine's inability to prevent it from interfering with the landings and subsequent re-supply of those troops already ashore.