WWII in Europe: Battle of Britain & Operation Barbarossa

Why did Hitler assume that his air-force could defeat Britain alone?
The Luftwaffe had no airforce to oppose it, which would cause the Royal Navy to be exposed and thus prompt Britain to "come to the negotiating table."

What factor, in your opinion, was the most important in Britain's victory in the Battle of Britain?
The most important factor in Britain's victory was their revolutionary warning system: radar. This also allowed Britain to devote more planes to the war rather than expending them on patrol runs.

Why did Hitler invade into the Balkans and why was this significant?
Hitler invaded the Balkans because Italy's offensives were failing. The entry of the German's into the Balkan war was significant because its strength in attacking Greece diverted British troops from the war in North Africa, weakening the Allied line there. Additionally, and unfortunately for the Axis powers, Hitler's contribution in the Balkans delayed his plan to attack USSR by six crucial weeks.

What were Hitler's motives for attacking the USSR?
Seizing the USSR would help Hitler achieve Lebensraum ("living space") for the German race. The East also contained valuable natural resources, such as oil. What's more, Hitler wished to demolish the "inferior" Slavs and their Communism.

What factors slowed the initial blitzkrieg of Russia?
The heavy rains of October and harsh winters greatly impeded Germany's Operation Barbarossa. Germany's failure to prepare for the brutal Russian weather was due in part to their overconfidence, believing they would be victorious before the crux of winter set in. In the end, neither Leningrad or Moscow were seized.