In the spring of 1940, after Germany's defeat of France, Hitler believed that Britain would ask for terms but if not then the country could be successfully invaded.
Planning began for Operation Seelöwe (Sea Lion), in July 1940, following Britain's rejection of Hitler's final offer of a negotiated peace settlement. Hitler wanted an immediate invasion to prevent the British army recovering from its defeat in France. All this was to be completed by 10 August 1940.
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.
No one knows whether Sea Lion would have succeeded or not had it been launched. S3#115 OSL asks..."what if..."