Answer: The Nazis believed that the Aryan race was the first and foremost superior race. This believe led to the idea that other lesser race were meant to serve the Aryan race and were meant to be preserved (except the jews and gypsies) to some extent to serve this purpose. Nevertheless, space was needed to be occupied by the Aryan race and hence their invasion and occupation of Poland and plans for further occupation.
German conquest of Poland was swift and effective which boosted their morales and also showed their strength to the enemies and their Allies. This was not expected.