Devils Adovate Argument
What was the Axis worst mistake in WWII?
I'll argue about each
1. Italian Invasion of Greece
No excuses. The Greek Army, in that terrain, was actually better man-for-man and unit-for-unit. They ruined themselves invading Albania, where the LOCs were horrific, and left their own right flank as a gaping hole for the German Army.
OTOH, there is a critical stretch of a rail line running through Eastern Thrace that if severed by the Greeks cuts Hitler off from his most critical resource, Turkish chrome. Had the Allies never bombed Germany's factories, thereby reducing German war production, Germany would have used up their chrome and seen their ability to manufacture weapons collapse by mid-1944.
Even OTL, January 1st 1946 was the projected date for Germany's exhaustion of the last of its strategic reserves of chrome. My source is Speer's memoirs. Much of it self-serving, but on technical levels pretty accurate.
2. German Invasion of Russia, aka Operation Barbarossa
Unavoidable. If Hitler waits until the Soviet's Second Five Year Plan is completed, the effects of Stalin's purges will mostly have been alleviated. Hitler may still get some measure of surprise, but he'll still be facing a much stronger Red Army. For Hitler, it was 1941 or nothing. If he'd invaded in 1942 or later, its unlikely the Germans get past the Dneipr-Smolensk-Leningrad line, but facing a much deeper in depth defending Red Army.
Forget Typhoon. Some form of Case Blue could happen, but be worse for the Germans. The Slaughter of Kharkov is still likely though.
.Japanese Attack on Pearl Harbor.
Japan's history of launching surprise attacks against potentially more powerful but politically weaker opponents certainly worked against them in WWII. Its a truism that Westerners have a poor understanding of non-Western cultures. Its easy to forget that non-Western cultures can be just as ignorant of us.


That's pretty much the Japanese problem in their strategic thinking regarding the West.
They were right about the Netherlands and the British Empire being too distracted to give Japan their full attention. But their inability to understand the domestic political ramifications upon the American People by a sneak attack (remember, that famous cable's language never contained an actual war declaration) while
still involved in high level peace negotiations spoke volumes about Japan's ignorance. While they may have been surprised at the ferocity of America's anger at the attack on Pearl Harbor, the Japanese never seemed to really take it too seriously.
As in, that the Japanese warlords never thought this would mean that the USA would NEVER negotiate with Japan now.
4. Hitler's Declaration of War on the United States
I strongly disagree, understanding that I am in a small minority here. Hitler, as a natural bully, was never going to let the other guy get in the first punch, even diplomatically. But it wasn't just mindless grandstanding by him. He had damn good reasons for his DoW against the USA, albeit all short-term ones. But those were the only choices he ever made anyway.
Consider the consequences if Hitler DOESN'T DoW America:
a) FDR can DoW Germany whenever he wants, at a time of his most advantageous choosing. Probably after mobilization is fully on track and the divisions are starting to complete their training, and the USN has stepped up its "limited war" against the U-Boats. Personally, I don't believe this could happen, whatever the public opinion polls may have reported between Pearl Harbor and Hitler's DoW. There certainly would have been a far stronger demand for a "Japan First" strategy, starting with Admiral King, who would enjoy a much stronger wind in his sails even with America DoWing Germany ITTL.
b) No German DoW means that at the very least, the USA is now an Associate Power in WWII. Like the USSR, neutral in one theater, active in the other. But since the USA is a full military ally of both the Netherlands and the British Empire, there's nothing ITTL to prevent them from receiving unlimited Lend Lease from America, fully escorted in USN convoys. And God help any U-Boat who tries to stop our ships from sending aid to our (Free) Dutch and British friends. All sent to the UK for re-deployment worldwide. All in the name of fighting the "perfidious Japanese".

Of course, as sovereign nations the Dutch and British are well within their rights to do with their Lend Lease as they choose. Like re-crating them "Made In Britain" and "Gift from the British People to the Valiant Workers of the USSR".


This would just HAVE to be in Hitler's mind.
5. Hitler's Fixation on Wonder Weapons, aka Wunderwuffen
Meh. Boys and their toys. Corporals playing with toy soldiers. They made it possible to think that German superior weapons technology would save his bacon in the end. In the end, the Allies AND the Soviets had plenty of Wonder Weapons of their own. We just take things like advanced radar, good super tanks, and superior mass war manufacturing skills for granted.
6. Hitler's Underestimation of Navy or Sea Power
"On land, I am a lion, at sea I am a coward."
Adolph Hitler
Actually, Hitler understood his limitations on naval warfare. He just didn't understand the changes that took place on that subject in WWII.
7. Nazi Repression on the Occupied Territories
They were Nazis, not nationalists. Would they ever have gotten so far pre-WWII if they lacked the criminal mentality of Nazis? Even the Soviets had their limits. Nazis did not.
8. The Inability of the Axis to Get Spain to join in the fight
I split these to make the distinction, because they are different cases. Spain was a Pro-Axis neutral. Turkey was a strict neutral.
Spain's Franco very much wanted a good sized chunk of French North West Africa, but that would put him at loggerheads with Mussolini and his recognized sphere of influence. He wanted Gibraltar, which would have been fine with the Axis, provided he had no objection to the colony being flooded with German troops, aircraft, and submarines. But he also wanted to make his army every bit as armed as the German Army, and Hitler could hardly spare all those weapons. He needed them for his own forces.
Without acceding to all of Franco's demands (which Hitler will never do) Spain isn't joining the Axis short of Britain's complete collapse.
9. The inability to get Turkey to join in the fight
As described in detail by others, Turkey was extremely unenthusiastic about the idea of entering WWII. And unlike Franco possibly responding to bribery, the Turks were more concerned about national survival, bordering on Russia as they were. Unlike Spain though, if the war news gets bad enough for one side or the other, Turkey will DoW the losing side to save themselves. They said as much to the Germans during their long tide of retreat on the Eastern Front.
Mind, Turkey didn't DoW Germany until February 23rd, 1945. By which time the rush of neutrals to declare war on the Axis had become a stampede. You had to be at war with the Axis if you were to join the United Nations.