biggest mistake that lost the axis the war?

Definitely, the invasion of USSR. By deconstructing the factories quickly as they backed up, the Russians were able to rebuild the factories behind the frontline. The new weaponry came out, such as La-5 and La-7 - by far, the best fighter-plane and storm-plane of WWII, respectively. I won't even mention T-34 - that's a well-known legend to all. By the time Normandy invasion happened, Russia was already invading Germany and, overall, Russians took off over 85% of the German forces. Thanks to Americans for holding the Japanese off at the Pacific.

 

And maybe it was the Japanese telling their troops to commit suicide instead of actually trying to defeat the Americans.

 

Like many war leaders have discovered at the cost of many lives, fighting a two front war is usually a bad idea. The Germans did not learn from WWI and repeated the mistake during WWII.

 

WWII situation was quite different from WWI, although there were similarities.

In WWI, Germans were not able to take out France, and the western front were on European mainland, which made it an immediate threat to Germany.

In WWII, the Germans were able to quickly wipe out everyone in western Europe. Although UK were still holding out cross the strait, they were in no position to launch any offensive onto the western Europe continent. It made Hitler felt there was no real threat on the western front, thus there was not a real western front at all.

Hilter also felt that indeed no military commander would want a two-front war, thus it was a total surprise attack on USSR because nobody expected it. It worked out well at the beginning, as the Germans destroyed bulk of Russian troops in the early months of the war.

So I wouldn't call invading USSR the decision itself is the worst mistake of the Germans.

 

Hitler and his allies made several serious mistakes, but it's really hard to say exactly which one brought about their defeat. His generals were absolutely convinced that fighting a war on two fronts simultaneously was a disaster in the making, it was written in Lesson One in the military textbooks that the Prussian elite studied in the military academies from the time of Clausewitz.
There are many good mistakes outlined by posters above, but I think there are a few we haven't mentioned that maybe deserve looking at
1) At the height of the Battle of Britain in August 1940, when we were basically against the ropes, Hitler switched from attacking aerodromes to cities. Bad for cities obviously, but a godsend to the RAF that was losing a lot of infrastructure on the ground much faster than could be patched up again. We would have lost the battle if the Luftwaffe had kept on remorselessy hammering away at our squadrons' bases.
2) Hitler not listening to his admirals to build more submarines to cut desperately needed supplies and troops fro the US and Canada.
and 3) The German intelligence network falling hook line and sinker for false information about the date and time of the invasion of France in 1944. Ever heard of FUSAG? One of the most brilliant acts of military deception.

 

Hilter was a genius as a political man, he was great at getting people following him and worship him, but he shouldn't have interfered with detailed battle plans even though he made all the foreign policies to start all the wars in the first place.


The problem is Hilter wasn't a military leader, and he thought so highly of himself. If he let his generals to make battle plans Germans would have done a lot better in WWII. Instead, all the generals have to get his approval on every little thing. Hitler was one major reason for Germans not be able to do as well as they could.

For the bombing of cities during Battle of Britain, Hitler simply wanted revenue for the bombs dropped on Berlin. The whole thing was an accident when a few German bombers accidentally dropped bombs in London which resulted in RAF to retaliate against Berlin.

 

I agree with earlier posters here; the Axis powers made two fatal mistakes in their strategy. One was to open up another front with Russia. Hitler ignored the warnings of his military advisers and turned his sights to the invasion and defeat of the Soviet Union--with catastrophic results.

The Germans were completely unequipped to handle the Russian winter--added to which, the Soviet Red Army was eager to crush the Nazi war machine, because of the torture that Hitler's army had already inflicted on the Russian people.

The other mistake was made by Japan in bombing Pearl Harbor. This brought the US into the war--opening another front for Hitler to have to contend with.

Another miscalculation was the endurance of the Royal Air Force. Hitler thought that the British would capitulate; he had no idea of their stamina and enthralling determination. That's why the Battle of Britain was such a defeat for the Luftwaffe.