[Gnomey]

I believe the biggest mistakes of WW2 were: Hitler's declaration of war on the USA in support of Japan when the Pact of Steel did not include a clause saying that one of the allies would support the other if they became involved in war, if this was the case then Japan would have had to enter the war in 1939 and not 1941 unless the broke the clause in the Pact with Germany and Italy. The other big mistakes of WW2 is Hitler's invasion of the USSR (Barbarossa) as well as his failure to win the battle of Britain if he had won he would have had only to fight the USSR and not the British Empire, America and the USSR probably 3 of the 5 most powerfully countries at the time (with Germany and Japan the other 2) an almost impossible situation to win.