In 1914 the Germans got bogged down in a two-front war. In 1939-1940 the Germans did not have to worry about the East (once they had crushed Poland) and were able to subjugate France, Belgium, the Netherlands, Luxembourg, Denmark, and Norway...while getting raw materials from the USSR.
Germany forgot one of the important facts of life (and paid dearly as a result). No need to buy the cow, when you’re getting the milk for free.