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.