I emphatically advice against reading Mein Kampf. That book was a propaganda product written at a certain point of Hitler's political career and almost all the details on his life are false and his positions fit only in that context, which if you are not familiar with it does not help gain a deeper understanding of Nazism and its ideology.

Adolf Hitler in his political convictions was the product of the pre-WWI Austrian and post-WWI German political context, which were ripe with tropes about Jewish conspiracy and the dangers of Bolshevism.

To completely understand the Jewish Conspiracy trope, we actually need to start with modernity itself. The Enlightenment and the onslaught of modernity following its earlier thinkers but especially the French Revolution had a profound impact on the thinking of the 19th century. With God being out of the game as the factor upon which the course of history and the legitimacy of power could be rested, discursive pressure formed to find new explanations for why the world was the way it was, why the people in it were different from each other, and what gave political power and order its legitimacy.

To solve this conundrum, various people formulated different answers. One you might be familiar with was Marxism, in the sense that Marx posed that the underlying force of history was class conflict and the legitimacy of power ultimately derived from the ownership of the means of production (simplified version here). But another and for this question very pertinent answer was also found in Social Darwinism. Social Darwinism wants to apply the concepts of survival of the fittest and natural selection to society and politics. In the age of the rise of nationalism, which saw nations resp. the according races as the actors in the historical process (like Marx viewed classes), the theory of Social Darwinism was combined with the theory of races as the historical actors and created what in essence became the völkisch ideology.

Now where do the Jews fit into and what does this have to do with some sort of alleged conspiracy, you might be asking. Well, in the tradition of völkisch thought as formulated by thinkers such as Gobineau and Houston Steward Chamberlain races as the main historical actors were seen as acting through the nation, the latter being basically their tool or outlet to compete in Social Darwinist competition between them. The Jews thought of as a race had no nation - seen as their own race, which dates back to them being imperial subject and older stereotypes of them as "the other" - but were a "race" that acted internationally rather than nationally. In order to be able to compete within the racial conflict them having no nation were seen as acting in a conspiratorial manner. Chamberlain e.g. made them out to be the controlling parasites behind political action and order that was seen as anti-national such as the Catholic Church or the Habsburg Empire. The anti-Semitism that formed here in the later stages of the 19th century is in effect a ideology of conspiracy, alleging a Jewish conspiracy in order to weaken their racial competitors.

The clearest example of such a way of thinking can be found in the Protocols of the Elders of Zion, a political treatise produced by the Tsarist Secret Police at some point in 1904/05 that alleges to be the minutes of a meeting of the leaders of the Jewish world conspiracy where they discuss their plans to get rid of all the world's nations and take over the world. Despite these protocols being debunked as a forgery really quick, they had a huge impact on many anti-Semitic and völkisch thinkers in Europe, not at least for some in the Habsburg empire such as Jörg Lanz von Liebenfels and others which were most likely read by the young Hitler.

The whole trope of the Jewish conspiracy as formulated by völkisch thought took on a whole new importance with the end of WWI, the Bolshevik revolution, and the subsequent attempts at revolution in Germany and elsewhere.

The defeat of the Central powers were seen by many of its soldiers and ardent supporters not as a military defeat but as a "stab in the back". The way the war ended in Germany with revolts of soldiers and the deposition of the monarchy by Social Democrats was the foundation for this myth that in essence revolved around Germany not being defeated by the Entente but by the enemies within. The trope of the enemy within being Jews and leftists had been brewing for a long time (see the Jew count of the German army in 1916/17) but really came to the forefront with the defeat. What followed compounded this further. The violence of revolution and counter-revolution as well as the treaty of Versaille lead to many völkisch inclined thinkers and political actors believing that Germany's defeat and the subsequent peace terms could only be explained by a concerted act of the jewish conspiracy leading to internal enemies stabbing Germany in the back, threatening the very German way of life through Bolshevism and preparing the Jewish-Bolshevik takeover of Germany by making it defenseless through the Versaille treaty.

Democracy seen as faulty and antithetical to the German racial character and communism as an essential anti-national movement were both shunned by these völkisch ideologues and explained through a concerted effort by a conspiracy of the anti-national "race", the Jews. This was the very core idea of völkisch thought and of Nazi Weltanschauung. In the end, for Hitler and many of his followers it was the only way to explain the state of the world because it hinged on this Social Darwinist, ultra-nationalist view of history being a history of races competing for power and supremacy.

Hitler was politicized and right at home in these contexts like so many others around the same time. Where he personally picked up these ideas and lines of thinking is impossible to say by now but the best Hitler biography, Ian Kershaw's Hitler, places it right after WWI when he was sent as a military spy into the nascent DAP (the later NSDAP) party to check out what they were up to.

Sources:

• Chrisoph Dieckmann: Jüdischer Bolschewismus 1917 bis 1921. In: Fritz Bauer Jahrbuch 2012.

• Robert Gerwarth: The Central European Counter-Revolutionary: Paramilitary Violence in Germany, Austria, and Hungary after the Great War.

• Andre Gerrits: Anti-Semitism and Anti-Communism in Easter Europe.

• Peter Pulzer: The rise of political anti-Semitism in Germany and Austria.

• Ian Kershaw: Hitler.