The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

Japan and Italy were not prepared to go to war with the Western powers –
not at that time, at any rate. So Germany had turned to the Soviet Union
instead.
Hitler suspected Britain and France’s pledge to Poland was nothing
more than bluff. However, in case it was not, it was clear that he had to
avoid an alliance between France and Britain in the west and Russia in the
east, because as he moved to Poland, it would be easy for Stalin to start
becoming nervous about German intentions. In 1939, Germany’s armed
forces were big enough to risk war with Poland, but not the Soviet Union.
It was von Ribbentrop who was the prime mover in approaching the
Russians and securing a non-aggression pact, eventually signed on 23
August 1939. That Germany was prepared to sign a treaty with an
implacable enemy was certainly cynical in the extreme, but hardly unique.
It meant Hitler could invade Poland and secure the corridor in the safe
knowledge that the Soviet Union, at any rate, would not object.
Despite the German military might carefully depicted on the newsreels,
Germany was only just strong enough to attack humble Poland in
September 1939. Her air force was her biggest asset, but the army was
under-trained and, worst of all, she faced a massive ammunition shortage.
Yet Hitler was in a hurry; time, he believed, was not on Germany’s side,
because her economy was faltering and, thanks to the inherent shortages of
fuel and, especially, iron ore, his rearmaments programme had almost
ground to a halt.
It was after Munich that Hitler had begun to realize that a showdown
with the West was probably inevitable. His long-term strategy was still very
much the establishment of Lebensraum in the east and the conquest of the
Soviet Union, but the threat of the Western powers would clearly have to be
dealt with first. In September, Generalmajor Georg Thomas, the Wehrmacht
chief economist, was told to prepare for war against Britain in 1942. A
fortnight later, on 14 October 1938, Göring announced a further
rearmament programme that was to dwarf the earlier, yet still considerable,
military growth. The Luftwaffe was to increase fivefold to some 21,750
planes, including 7,000 Ju 88s and more than 800 He 177 heavy bombers.
The navy was to begin a fleet-building programme, called the Z Plan, which
would include six new battleships and hundreds of U-boats and other
vessels that would make it comparable in size to the Royal Navy within six