A lot of people talk about Hitler's supposed failures of military strategies during World War II, citing his declaration of war on the U.S. and his 1941 invasion of Russia. However let us talk about these two supposed 'blunders' of military strategy.
First of all, considering that FDR was already going to war against German U-boats in the Atlantic in 1940 Hitler was fully within the right to declare war on America. Secondly, FDR was given by British intelligence a supposed map of "nαzι plans" in this hemisphere, all lies cooked up by British intelligence to lure America into war. Hitler would later cite this map as an American provocation in his declaration of war. The point of mentioning this is that FDR and his cabinet was fully prepared to go to war against Germany by any means necessary so Hitler was actually just accepting the inevitable.
To the second point of the 1941 invasion of Russia Hitler did not invade Russia for Lebensraum or to crush ʝʊdɛօ-Bolshevism but to knock out the one country (besides America) Britain had the most hope in. Britian still clinged onto the hope that Russia would crush Germany from the east which is why Britain largely continued the war against Germany. Hitler's invasion of Russia was to crush the nation so that Britain would also surrender. Hitler offered Britain peace twice, both times she refused, and so Hitler hoped that by crushing Russia Britain would finally accept surrender.
Hitler's biggest blunder in my opinion of strategy was his constant rescuing of Mussolini in Italy, by use of German troops, wasting time and resources.