By far his greatest weakness was his belief that he was a military and strategic genius. He wasn’t. He had some startling early successes, but the longer the war went on, the more out of touch he became with the realities on the ground. He ignored the evidence because he was so certain that he was right. He gave orders that simply could not be obeyed.
His obsession with ethnic and genetic purity, and wiping out all the Jews, gays, disabled people, gypsies, mentally ill people and so on also obstructed the war effort. Valuable coal was consumed, and valuable trains and railway lines occupied, transporting people to the death camps when they should have been used to carry war materiel to the front. Hitler had lost any sense of proportion. I don’t know if the Holocaust actually cost him the war, but it certainly didn’t help.
The defeat in Stalingrad and the first offensives of the Soviets he didn’t let his Generals regroup their armies because then they would temporary let the enemy take their land (I know…). But another weakness was to overestimate the situation. In fact the German soldiers didn’t have any warm winter uniforms like the Soviets did, theirs were black, which doesn’t fit fashionable with snow so well, especially if you want to survive. So his soldiers had a big disadvantage to the Soviets, but after a time they got some kind of sheet to pull over.
Another dumb thing on the eastern front was to not attack Moscow as fast as possible. He could have used many if his tanks to attack the city as fast as possible, but instead spared some of his tanks for later campaigns, and so was forced back 30 kilometres from Moscow.
And also he wasted bombs and airplanes to bomb the UK (which he could have used against enemy armies) although it didn’t make much of a difference since much of the military equipment, that were produced in the cities, later came from the US and the bombing of civilians also didn’t help winning the war (historians said that).
Another weakness of Hitler’s military leadership was his fixation with ‘miracle weapons’, which he believed would transform the war in the East. These included the Tiger and Panther tanks (which he ordered to be rushed into action at the Battle of Kursk without adequate trials). Although these vehicles proved formidable, they could never be produced in enough numbers to defeat the hordes of T-34s turned out by the Soviet Union.
After the July Bomb Plot of 1944, in which Count Claus von Stauffenberg’s briefcase bomb narrowly failed to assassinate Hitler in his military headquarters, the Führer’s relationship with his army generals broke down almost completely. For the rest of the war, as Nazi Germany faced inevitable defeat, Hitler could only advocate fanatical resistance and order futile, doomed counter-attacks, such as Operation Solstice in Pomerania in February 1945.
In 1945 Hitler’s determination to fight to the bitter end condemned millions to a violent and pointless death. This was never more true than in the final Battle of Berlin, which produced more than half a million casualties and brought incalculable suffering to the civilian population.