Its not on the list but I'll go for the defense or rather the lack of defense by the British at Singapore there was adequate men and equipment to have fought off the Japanese but the leadership and planning where appalling.
This is true even to the extent that the Brittish were allowing Commonwealth troops to land after it was obvious that they were going to relinquish the position to the enemy. This resulted in Thousands of combat ready troops to be tortured to death either in Changi prison or building a railway to nowhere in the jungles of Burma.
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If I had to pick any other I would say Barbarossa soon as Hitler started on the Russians the whole out come of WW2 was sealed including that of the Japanese. Germany could not defeat Russia and the western powers simultaneously. [/QUOTE]
This i also have to agree with so long as the russian campaign as a whole is considered i mean the loss of 600 000 combat experienced soldiers was no small contribution to the whole downfall of the Reich.
I would however like to bring another of Hitler's monumental and numerous mistakes. This being the demand he placed on Herman Goering to flatten London rather than allowing the commander of his Air force to do what he knew was needed, destroy the RAF's capacity to field defensive A/C and remove their ability to controll the sky.
This one domino i beleive if it had fallen would of resulted in the result of WWII being very different indeed. Air Marshal Dowding himself conceded to Churchill that he only expected the Fighter command Airfields to remain operational for another three days more during the heaviest period of the bombing of the Fighter fields. The change in tactics at Hitler's behest allowed the breathing space the fighter squadrons needed to regather and face the threat streaming over from occupied France