World War II  was a long and bloody war that lasted for six years. Officially beginning on September 1, 1939 when Germany invaded Poland, World War II lasted until both the Germans and the Japanese had surrendered to the Allies in 1945.The pact signed in 1939 by Germany and the Soviet Union that enabled a one-front war when Germany started World War II.When World War I ended, the experience of it seemed to vindicate the power of the defensive over the offensive. It was widely believed that a superiority in numbers of at least three to one was required for a successful offensive.

The Germans learned this crucial, though subtle, lesson from World War I. The
Allies on the other hand felt that their victory confirmed their methods,
weapons, and leadership, and in the interwar period the French and British
armies were slow to introduce new weapons, methods, and doctrines. Consequently,
in 1939 the British Army did not have a single armored division, and the French
tanks were distributed in small packets throughout the infantry divisions. The
Germans, by contrast, began to develop large tank formations on an effective
basis after their rearmament program began in 1935.