Hitler and the Oberkommando des Heeres (OKW) initially rejected the plan. However, on 2 July, Hitler issued a Supreme Command Directive instructing all three Services to initiate preparations for an invasion of England. This was followed by a further Supreme Command Directive on 16 July which instructed that preparations for landings between Ramsgate and the area west of the Isle of Wight be completed by 10 August 1940.

Throughout July and August the German Supreme Command continued to revise the plan and on 27 August the Führer agreed to an invasion on a narrower front between Beachy Head and Deal together with diversionary tactics by the Kriegsmarine to draw Royal Navy shipping into the North Sea and North Atlantic. Both Hitler and the Supreme Command agreed, however, that air supremacy was the principal condition for a successful crossing and in July the Luftwaffe launched Operation Eagle (Unternehmen Adler). The ensuing Battle of Britain saw the Luftwaffe fail to achieve adequate air supremacy for Sealion to be launched. D-Day was postponed, at the request of the Naval Staff, from 15 September to 21 September due to delays in preparations, and on 17 September Hitler postponed Sealion indefinitely on the grounds that essential conditions did not exist. On 9 January 1941 Hitler issued the order to discontinue preparations for 'Sealion'.

Opinion remains divided as to whether 'Sealion' would have been successful or not had it been launched. What is undeniable is that without air supremacy and command of the sea the decision not to launch was the right one. The JSCSC Archive contains four contemporary documents relating to 'Sealion' which focus primarily on the British assessment of the threat.