Yes it was possible. Early in the precepts of the war, Germany could have cut off Britain's retreat to their home island. If they had done this, it would have been too devastating a blow to the Brits and they would not had the ability to handle Germany's invasion. Basically they started off to soft and let things get out of hand. If they had secured Britain and the west they could have focused on Russia. Once Germany gets to the oil supplies blocked off by Moscow, it's game over.