Overy points out how the advance to the East & Lebensraum was alway to the forefront of Hitlers plans moving along step by step...
In Mein Kampf he openly stated that the future of Germany "has to lie in the acquisition of land in the East at the expense of Russia.
In Zweites Buch he adds the U.S. to his 'Jewish/Bolshevik' conspiracy to explain why Germany must have the territory in the East.
At his first meeting with all of the leading Generals and Admirals of the Reich in February 1933, just four days after he became Chancellor, Hitler spoke of "conquest of Lebensraum in the East and its ruthless Germanization" as his ultimate foreign policy objectives.
He reinforces the views he'd developed in Mein Kampf and his 2nd book.
Nothing had changed his belief that struggling for Lebensraum was Germanys' only salvation.
The demand for Lebensraum was not just a Nazi pipe dream. At the London Economic Conference of 1933, the head of the German delegation, the Economics Minister Dr. Alfred Hugenberg of the German National People's Party, put forth a programme of German colonial expansion in both Africa and Eastern Europe as the best way of ending the Great Depression, which created a major storm at the conference.
In 1936 Walter Darre, his agriculture minister said to a group of impotent RNS officials, 'the national area for settlement by the German people is the territory to the East of the Reich bound up to the Urals, bordered in the South by the Caucasus. we will settle this space according to the law that a superior people always has the right to control and to own the land of an inferior people.'
In June 1940 Hitler told his generals that the victories in western Europe "finally freed his hands for his important real task: the showdown with Bolshevism"
On the 31st of July 1940 he gets his Generals together at his alpine retreat, the Berghof near Berchtesgaden, and he announces to them that they must prepare for an invasion of the Soviet Union the next year.
On 5 December 1940, Hitler received military plans for the invasion, and approved them all, with the start scheduled for May 15th 1941.
That was converted then into signed Directive 21 on December 18 1940.
The war in the East was always going to happen and it had to happen sooner rather than later because of the pressure of time that Hitler felt very acutely, and unfortunately in June 1941 he put into action exactly what he wrote & spoke about.