Pure speculation, but maybe it’s calculated aggression based on an assumption that the west doesn’t have an appetite for war [with Russia] and will avoid doing so if possible.
How so? Hitler got away with it. The Allies declared war on him only nine months later when he also invaded Poland.
And even then, they only did it half-heartedly, and the French pretty much threw away their weapons when the first German panzers arrived. Hitler knew that he had the stronger will to fight. Had it not been for a few other factors (Churchill, Russian winter, and the Hitler's obsession with the jews), he could have taken the whole of Europe.
