There was this idea that arose in post WWI Germany called the 'stabbed in the back myth'.
Germany was in really rough shape by the end of WWI; its economy was wrecked, its population was on the verge of starvation, and its military was running of of reserves and material with which to fight. The Allied Powers had also had a rough time but the entrance of the US into the war meant that any German attempt at a "knock out" blow before they totally collapsed was basically out the window. The realization that the war was effectively over lead to a period of political turmoil inside Germany which ultimately lead to the signing of the armistice that ended WWI.
However, there wasn't really a big final battle where the German Army was driven off the battlefield in defeat. They were still in fighting condition at least in the short run and were still on foreign soil when the war ended. This had certain consequences after the war.
If you were a German soldier who had fought in the war (like Hitler for example) the idea that the war had been all a big mistake and that all the terrible stuff that had happened to you and your friends had been for nothing was a bitter pill to swallow. It was a lot easier to believe that the war could have been won relatively quickly if only they hadn't been "stabbed in the back" on the home front by unpatriotic socialists and Jews.
This idea of the undefeated German Army being stabbed in the back wasn't limited to front line soldiers. Someone like Erich Ludendorff, who had been more or less in charge of Germany's war effort, needed a way to explain what had happened during the war. The idea that "disloyal" civilian elements had sabotaged his sound military policies was an idea that Ludendorff liked, and was something that he and people like him did a lot to spread around after the war.
In Hitler's own words:
And then one day, suddenly and unexpectedly, the calamity descended. Sailors arrived in trucks and proclaimed the revolution; a few Jewish youths were the 'leaders' in this struggle for the 'freedom, beauty, and dignity' of our national existence. None of them had been at the front. By way of a so-called 'gonorrhoea hospital,' the three Orientals had been sent back home from their second-line base. Now they raised the red rag in the homeland.
...
My first hope was still that this high treason might still be a more or less local affair. I also tried to bolster up a few comrades in this view. Particularly my Bavarian friends in the hospital were more than accessible to this. The mood there was anything but 'revolutionary.' I could not imagine that the madness would break out in Munich, too. Loyalty to the venerable House of Wittelsbach seemed to me stronger, after all, than the will of a few Jews. Thus I could not help but believe that this was merely a Putsch on the part of the navy and would be crushed in the next few days. The next few days came and with them the most terrible certainty of my life. The rumors became more and more oppressive. What I had taken for a local affair was now said to be a general revolution. To this was added the disgraceful news from the front. They wanted to capitulate. Was such a thing really possible?
...
Since the day when I had stood at my mother's grave, I had not wept. When in my youth Fate seized me with merciless hardness, my defiance mounted. When in the long war years Death snatched so many a dear comrade and friend from our ranks, it would have seemed to me almost a sin to complain- after all, were they not dying for Germany? And when at length the creeping gas-in the last days of the dreadful struggle- attacked me, too, and began to gnaw at my eyes, and beneath the fear of going blind forever, I nearly lost heart for a moment, the voice of my conscience thundered at me: Miserable wretch, are you going to cry when thousands are a hundred times worse off than you! And so I bore my lot in dull silence. But now I could not help it. Only now did I see how all personal suffering vanishes in comparison with the misfortune of the fatherland. And so it had all been in vain. In vain all the sacrifices and privations; in vain the hunger and thirst of months which were often endless; in vain the hours in which, with mortal fear clutching at our hearts, we nevertheless did our duty; and in vain the death of two millions who died. Would not the graves of all the hundreds of thousands open, the graves of those who with faith in the fatherland had marched forth never to return? Would they not open and send the silent mud- and blood-covered heroes back as spirits of vengeance to the homeland which had cheated them with such mockery of the highest sacrifice which a man can make to his people in this world? Had they died for is, the soldiers of August and September, 1914? Was it for this that in the autumn of the same year the volunteer regiments marched after their old comrades? Was it for this that these boys of seventeen sank into the earth of Flanders? Was this the meaning of the sacrifice which the German mother made to the fatherland when with sore heart she let her best-loved boys march off, never to see them again? Did all this happen only so that a gang of wretched criminals could lay hands on the fatherland?
...
Kaiser William II was the first German Emperor to hold out a conciliatory hand to the leaders of Marxism, without suspecting that scoundrels have no honor. While they still held the imperial hand in theirs, their other hand was reaching for the dagger. There is no making pacts with Jews; there can only be the hard: either-or. I, for my part, decided to go into politics.
Obviously any kind of speculation about what Hitler's deal actually was is only going to be speculation. It's possible that everything Hitler did was because he was just some fucked up asshole and the war didn't really have anything to do with how he turned out. I hope that what I've written helps to locate Hitler's antisemitism in the context of what was going on around him even if it can't fully explain why he did the things the did.
