[BluffTHIS!]

Most of the major mistakes of the axis have been mentioned above, but really a lot of them have not to do with the substance of those actions, but the precise ways they were carried out due to Hitler not being a totally rational leader. Hitler was capable of great daring and intuition that his generals were not. But what he did wrong was having made a strategic decision to invade Russia for example, was to insist on micro-managing the details instead of leaving them to the generals. If although insisting on a 2 front war he left the timing and tactical details to the generals, he could well have succeeded.

Thus if the invasion of Russian had been left to early the next spring with a larger cushion for error in the case they got bogged down so that winter wouldn't have been pressing on as soon, then he could have more likely suceeded. And as noted above his not allowing his commanders to act quickly without prior approval to commit their armor forces when they saw fit, particularly when they could have had a better chance of stopping the allies on D-Day by quick action, he often doomed his stratgic choices to failure.

In regards to Germany making a mistake in declaring war on the US, at the point it was made it was not really a mistake. The real mistake was indeed the Battle of Britain and not attempting to leave Britain alone and hope as was reasonably possible, that the British public would not support continuing the war if they themselves were secure in their island. Since Germany had made that mistake, then bringing the US into the war wasn't really for one reason that applies to Japan as well. And that reason is that both they and Japan had internal lines of supply and communication which the allies did not. Had the allies not in fact been able to break German codes, the U-boat war could have effectively stymied US efforts to supply it's allies and themselves in Europe.

However, both Germany and Japan suffered from an inability to threaten the United States homeland directly except for the Japanese attacks on Hawaii and the Aleutians. Thus other than killing as many American soldiers as they could, they could not take the war to the US citizens and force the US to spend much more of its resources on self-defense.

So the biggest mistake for Japan was the attack on Pearl Harbor without also an ability to follow it up closely with an invasion and occupation of same in order to both give it naval and air bases that could threaten the US mainland and also deny those bases to the United States.