The question is what "would have happened if it had gone ahead", not "was it possible". You need to be a bit more specific. (A) Q:What would have happened if the invasion had gone ahead at the end of September or the beginning of October 1940 (the last possible planned dates) - without changing anything? A: a British victory, the loss of the best troops (and generals) in the German army, a serious blow to German prestige, and a shorter war. The resistance movements in the Continent would have been given a lot of encouragement and become active earlier. The US would have provided significant help sooner. Hitler's position may even have been threatened. Help from neutrals to Germany would have dried up or at least be less forthcoming. Without removing the RAF, the invasion might have got its first wave ashore, but then the second and subsequent waves would have been unable to prevent RN intervention. Also, the supplies would be interdicted by the British air and sea forces. (B) Q: What would have happened if the invasion had gone ahead (possibly at an earlier date), as suggested above, as the result of a planned follow-up to the invasion of France, and a planned assault on the RAF? A British victory is still possible, but even a costly German victory would have had serious ramifications. The Germans had serious plans to arrest large numbers of Britons who were jews, intellectuals, socialists, or opposed to Germany in any way. They also planned to deport a large portion of the British male population of military age to Europe as slave labour. There were thousands of refugees in Britian who had escaped from Germany and other parts of Europe - those that weren't interned outside Britain (many were put in British concentration camps) would have been sent to German style concentration camps at least. Press freedom, freedoms of speech and association were restricted by the British government during the war, but would have been completely absent under German occupation. Severe reprisals would have been taken against any British resistance. We are lucky the German high command was ruled by a bunch of school yard thugs instead of professionals, thus making the Sealion planning a mess. Goering refused to do any Sealion planning for the Luftwaffe and issued no comprehensive orders for the Luftwaffe until after the most likely invasion date. There wasn't even a proper, comprehensive plan to defeat the RAF, with attacks before August 13 being launched on the initiatve of local commanders. The German army and navy did everything they could to ensure the success of Sealion, but with Goering dragging his feet and Hitler (the only person able to force inter-service co-operation) not telling him to do what the Sealion plan said the Luftwaffe should do, it would have failed. You may like to view my (500 Mb) powerpoint presentation on Sealion here: https://drive.google.com/file/d/0B_JIBYcrqYoOcjVvQlJ3ZjJqdUU
