Several postponements took place through into September on the grounds that the essential conditions did not exist. The Luftwaffe’s failure to achieve air supremacy in the Battle of Britain forced Hitler to postpone Operation Sealion indefinitely. The operation was never formally cancelled.Click to see full answer. Considering this, why was Operation Sea Lion a failure?If British losses became severe, the RAF could simply have withdrawn northward and regrouped. It could then deploy when, or if, the Germans launched an invasion. Most historians agree Sea Lion would have failed regardless, because of the weaknesses of German sea power compared to the Royal Navy. when did Operation Sea Lion? September 1940 Beside this, when did Operation Sea Lion end? September 1940 What was the Operation Sea Lion plan?Operation Sea Lion (German: Unternehmen Seelöwe) was the plan to invade the United Kingdom by the Nazis during World War II. The plan began in 1940. However, Germany first had to control the sky and sea of the English Channel before a land invasion.