Delaying  attack on Russia
Hitler’s major blunder was delaying attack on Russia for two months,which gave enough time to Russia to regroup,he attached Russia in June 1941 with 4.5 million troops; According to experts he lost war with Russia because of number of reasons i.e underestimating the capabilities of Red army,failed logistic planning and cruel weather.

Genocide against Jews,gypsies
Hitler started genocides against Jews,gypsies at very wrong time,when the World War 2 was going on ,it diverted attention and resources of Germans from war also losing jews and gypsy recruits for war and also jewish scientists who could have been very useful in the war.

Holding back war production
Hitler stopped war plane production during the World War 2,but Britain was producing more war planes than Germany,which gave added advantage to Britain during the World War 2.

Germany Vs USA
The greatest blunder committed by Germany was not keeping Japan from attacking Pearl Harbor, thus bringing the United States into the War. Up to this point, the United States was generally neutral, but providing some aid to the Allies.

Failed to make an Atomic Bomb
America tested first atomic bomb in World war 2 on Japan in 1945,which proved superiority of USA on all other nations of the world;According to historians hitler had enough scientific and financial resources to make an atom bomb but he failed to do so.