“Nazi Germany had already conquered Poland, Norway, Denmark, the Low Countries and France. In the latter part of 1940, Britain was the only nation still trying to stop the advancing Germans.”
Operation Sea Lion was the plan for invading Britain through the English Channel. As Britain was the last big power still opposing Hitler in Europe he was very keen to invade it before putting his focus on Russia (formerly the U.S.S.R.).
To achieve Hitler’s invasion of Britain, the Germans had to eliminate the threat of the British Air Force in order for the invasion boats to cross the channel without harm from the