After France fell Nazi Germany was not sure what should be the next campaign. Operation Sealion which would have meant an invasion of Great Britain was considered but the Luftwaffe failing to achieve air superiority in the Battle of Britain was the final nail in the coffin of those already unrealistic plans (Britain also outmatched Germany significantly in naval power, critical for supplying any landing force on the British isles).

Hitler turned to the east and while some of his generals may have privately harbored some concerns about the chances of a victorious campaign about the Soviet Union certainly none of them voiced them. Hitler thought it was the right time to attack while Germany was at the height of its power. The belief that the war would be over in 4 months was of course completely unrealistic, German intelligence underestimated what the Russians had badly and the Germans lacked a clear overall plan. Initial successes increased optimism but this would soon change...