Following the German defeat of Russia with the Treaty of Brest-Litovsk, the German army began the Spring Offensive on the Western Front, and experienced a large amount of initial success, however as the fighting went on they found themselves in a compromising strategic position, as both sides of the war grew weary. The German government saw the writing on the wall, and while they might have been able to continue the fight, the Central Powers were collapsing around them, initiated by Bulgaria being the first nation to surrender. Jews had been blamed for a variety of issues during the war, such as strikes within arms-industry factories which were seen as weakening the war effort. Socialists throughout Germany, although they had initially supported the war, began to protest and attempt to undermine the war effort, seeing it as a rich man's war on the back of the poor, which caused a growing hatred among German patriots for both communism, and socialism. Following the German surrender after WW1, the Bavarian Soviet Republic appeared in the city of Munich, whose leaders were mostly Jewish, which only intensified German hatred for Jews, and allowed both socialists and Jews to be tied together as enemies of the German state, who wished to see them fail.
Essentially, Socialists were blamed for a large amount of trouble during the war on the home front, with a tone of anti-semitism already existing, and with many socialist leaders within Germany being Jewish, that allowed the two to be tied together as a scapegoat for Germany's defeat in WW1.
sources: Kolb, Eberhard (2005). The Weimar Republic.
Levy, Richard S. (2005). Antisemitism: A Historical Encyclopedia of Prejudice and Persecution.
Antisemitism had a long history in Europe before the wars. In the middle ages and as Christianity rose to power, it was influenced by the idea that Jews were responsible for the death of Jesus. Some Christians believed that Jews kidnapped Christian children to use as blood sacrifices during religious rituals (Source). Jews were banned from many jobs in the general population. One of the few jobs they could take was money managers and lenders. Jews were often harassed and ostracized, so generally kept to themselves. there are many other examples of religious persecution in Europe, but As the Renaissance bloomed and a merchant class arose, the Jewish population involved in money lending became very wealthy. This spawned a growing resentment and an underlying feeling of resentment. (Source- US Holocaust Museum)
As time went on, antisemitism was less prominent in Western Europe (especially compared to the east, including the Balkans and Russia) but there still was an underlying prejudice. The Dreyfus Affair was one of the most famous examples of antisemitism, where a Jewish French military officer was convicted of treason. He was ruined in the court of public opinion, largely because of his Jewish heritage. (Source) In the early 20th century, literature like the Protocol of the Elder's of Zion was distributed. This document claimed the be the Jewish plan for world domination. It was actually distributed throughout the world (Henry Ford printed 500,000 copies to pass out in the US). Hitler read and was heavily influenced by the document- it actually became mandatory reading in school for all children after the Nazi regime rose to power (Source- The Protocols of the Learned Elders of Zion ). He actually discusses the Book in Mein Kampf "...To what extent the whole existence of this people is based on a continuous lie is shown incomparably by the Protocols of the Wise Men of Zion, so infinitely hated by the Jews. They are based on a forgery, the Frankfurter Zeitung moans and screams once every week: the best proof that they are authentic. [...] the important thing is that with positively terrifying certainty they reveal the nature and activity of the Jewish people and expose their inner contexts as well as their ultimate final aims" (pages 307-308). "During the First World War the first signs of a relocation of German antisemitism appeared. From being a strong, local (peripheral) phenomenon, which sometimes had a racial character, with limited objectives, and which benefited certain social groups in the provinces, it became a national phenomenon.The first step took place in the political arena. The German Fatherland Party (Deutsche Vaterlandspartei), a right-wing antisemitic party which came into being during the First World War, as a result of the union of the various conservative, antisemitic, racial forces in Germany, preached an antisemitic racial ideology in the latter part of the war. The party provided the conceptual and organizational model for all the antisemitic and nationalist movements that arose after the war, and was led by figures like Wolfgang Kapp and Heinrich Class, who made a decisive contribution to undermining the legitimacy of the Weimar Republic in its formative stages." (Source, pg 10)
So with this we jump to the end of WWI. Germany is in RUIN. Millions are dead across the continent, the country has been razed, the rest of the world is demanding reparations for the cost of the war, the old government has toppled- it is a mess. Remember your MAIN causes for WWI (militarism, alliances, imperialism, and nationalism), these are still at play. Imperialism and the "white man's burden" can be linked to the belief that the Aryan race is superior. And Nationalism? A popular right wing theory called Dolchstoßlegende (stab in the back myth) was circulated saying basically that Germany did not lose the war militarily, citizens (Jews and communists) back home over threw the monarchy and ruined everything for them (check out Germany Between Two Wars: A Study of Propaganda and War-Guilt by Lindley Fraser for more on this). At this point, everyone in Germany wants an explanation for why this is happening to them. Why is inflation so terrible, why can't they buy food, why did their sons die, why was the rest of the world blaming them for this war? Mixed with the fear from the Bolshevik Revolution, people were ready for any explanation (Source- US Holocaust Museum). "Many scholars today agree that the hyper-inflation of the years 1922–23 resulted not only in money losing its value, but also in a devaluation of human life. The national humiliation, the defeat of Germany, the astronomical sums the Germans were forced to pay in reparations, the sense of insecurity and the massive unemployment which overtook Germany towards the end of the 1920s, the great fear of the extreme left and the almost continual atmosphere of civil war undermined the civil foundations, bourgeois values (Tugend) and Christian morality that had hitherto characterized various strata of German society." (Source, pg 13.
As the Nazi party rose to power, Hitler spoke of the greatness of the German people and gave them the Jews as a scapegoat for their problems. His speeches and writings spoke of a racial struggle. He used ideas that had been part of the European mindset for centuries. Antisemitism was alive and well in Europe and Hitler capitalized on the population's belief that they were a truly exceptional people who had been manipulated and scammed by these other people. He promoted the idea that the Jews were somehow stealing and mishandling money, which put Germany into economic ruin (Source). The NY times actually re-published a really interesting piece on their first profiles of Adolf Hitler which includes some reactions of people who heard his speech. He was rising to power, he needed something or someone to blame for the problems, and he needed a way to solve that problem. His final solution rose out of these beliefs.
I have a lot more sources on this, but I'm at work. When I get home I'll include a few interesting books to read and articles to check out. I had a professor in college who was one of the leading expert on Jews in Germany before the war so he gave me some really interesting stuff.
Edit- Adding some more sources to check out:
The Rise of Political Anti-Semitism in Germany and Austria, Revised Edition, Peter Pulzer
Hitler’s willing executioners by Daniel Jonah Goldhagen, Maurice Wohlgelernter
Roots of Hate: Anti-Semitism in Europe Before the Holocaust by William Brustein
It wasn't just Hitler who blamed the Jews for the German defeat in WWI, loads of people believed it as soon as 1919. The "stab-in-the-back"-myth originated right before the end of WWI.
Right before the end of WWI, general Erich Ludendorff was the most powerful man in Germany. After signing the Treaty of Brest-Litovsk with the newly formed Bolshevik government and thus ending Russia's participation in WWI, Germany stopped fighting on two fronts and could focus on the western front. Thus Ludendorff started planning the last big German offensive, the Spring Offensive.
Though initially succesful in terms of territorial gains, the German army outran their supply lines and their depleted units couldn't live off the land they conquered. The allies then launched the Hundred Days Offensive and defeated the German army. Ludendorff had a mental breakdown as a consequence.
Ludendorff realised Germany was defeated and created a civil government in order to get better peace terms (assuming a civil government would get better terms than a de facto military government) in october 1918. This civilian government continued to negotiate what he considered to be a humiliating armistice with Woodrow Wilson. Instead of blaming himself for losing the war, Ludendorff blamed the civilian government for signing the humiliating armistice.
After the signing of the treaty of Versailles, Ludendorff went in exile to Sweden and started numerous books about WWI. In these books he argued that Germany had actually fought a defensive war, and Kaiser Wilhelm II wasn't a good leader. He blamed social democrats and leftists for signing the Treaty of Versailles. Ludendorff and for sparking the civil unrest in 1918. Communists were blamed for radicalising the German soldiers returning from the Eastern front.
Ludendorff gets cause and consequence wrong here obviously. The war wasn't lost because the German army lacked morale to continue fighting, the German army lacked morale to continue fighting because the war was already lost at that point.
Instead Ludendorff blamed social democrats, leftists, and jews (the German empire has a long history of antisemitism) for sowing civil unrest in 1918 which according to him caused Germany to lose the war.
This myth ('the German army was undefeated in battle but was betrayed by unloyal elements') was propagated by conservative and nationalist elements in the early days of the Weimar republic. Ludendorff himself joined Hitler's NSDAP. Even Friedrich Ebert, who was president of the Weimar republic at that time, propagated the myth when he saluted return veterans saying "No enemy has defeated you". The myth was kept alive both because it was good for the morale of the army, and because the Weimar republic often needed to deal with uprisings of far left elements. Blaming them for losing the war was simply good propaganda.
In short, because the type of ethnic nationalism embraced by the Nazi Party excluded Jews from being "true Germans", because it was easy for the German far right to group together Jews and the political left into one monolithic enemy, and because far right nationalism needs an "enemy" (especially one which can be portrayed as an internal fifth column) in order to explain why a "superior" people can be defeated by "inferior" ones, e.g. non-Germans having defeated Germans in the First World War.
In more depth:
To begin with, the Dolchstoßlegende was already popular in Weimar Germany at the time the NSDAP (Nazi party) became politically relevant, meaning Hitler and the NSDAP were by far not the first ones to offer German Jews and others as scapegoats for Germany having lost the First World War. The specific brand of ultranationalism endemic to the far right asserts a theory of cultural supremacy -- and ethnic supremacy in some cases, of which the NSDAP definitely is one -- which must account for events and circumstances which disrupt the logical congruence of being allegedly better than everyone else while simultaneously having suffered defeat at the hands of an "inferior" people. Thus, ad-hoc theories like the "Stab-in-the-back myth" are common on the far right: essentially, it's impossible that the "superior" people could have been beaten in a stand-up fight, and thus the responsibility for the defeat lies with a fifth column that betrayed the nation from within. You'll even see this kind of rhetoric today being used by far-right groups in the United States and Europe ("we're being weakened by foreigners living in our midst and by members of our nation who don't share our patriotic values").
By the end of World War I, Germany had already seen several significant left-wing uprisings, including one immediately following WWI. As is true today, the far left and far right were bitter enemies and were quite literally fighting for the soul and future of the people of the German states. Given that many European Jews had historically been involved with various socialist, communist, and anarchist movements across Europe, and that Jews were a prominent part of social and economic life in Germany during the height of exclusionary ethnic nationalism, it seemed natural for the far right to group Jews and left-wingers together as part of the monolithic enemy that had "betrayed" Germany and caused its defeat in WWI.
With the transition from the medieval period to the modern era, the form of antisemitism in Europe had morphed from a theological antisemitism to an ethnic nationalist one. The racialist ethnic ultranationalism of the Nazi Party excluded Jews from being able to be "true" Germans. Despite the fact that Jews were heavily integrated into German society, they generally lived together and apart from non-Jewish Germans in insular communities. Many German Jews kept Jewish customs which were different from non-Jewish customs. This, in combination with scientific racism which was popular in the West at the time, provided plenty of justification for excluding Jews from what it meant (to the German right) to be German.
That actually wasn't a Hitler Original, but really came from attitudes all over the former aristocracy, capitalists, military leadership, and right-wing in general. It's what is known as the Dolchstoßlegende or "stab-in-the-back legend"
I'll give a shorter background just on the myth, so I don't stray too far from your question
See, many of the aforementioned individuals didn't feel as if Germany lost the war due to being out-fought (make no mistake, they were), because Germany wasn't fully invaded and occupied during the war. Instead, they blamed it on the civilians giving up on the war and causing all sorts of internal issues: unrest, strikes (especially in arms factories), murmurs of socialism and republicanism.
Of course, the biggest enemies (according to the right) here were the socialists, the communists, and the Jews, because not only was there significant representation of Jews in the revolution, Münchner Räterepublik, as well as the USPD (German Independent Social Democratic Party, a centrist party) and the KPD (German Communist Party). What this doesn't measure, however, is that the great majority of German Jews were well-off and middle class, and had a lot to lose in the revolutions, and were more conservative than revolutionary.
None of the facts matter, of course, and Jews were seen as cowards who avoided front-line combat during the war--again, against facts. In 1916, Adolf Wild von Hohenborn (the German minister of war) commissioned a "Jew Count" to prove that Jews lacked patriotism, and were rejected for service (popularly referred to ask Drückeberger, or shirkers) What he found was really the opposite. While the exact numbers vary based on source, between 80,000 to 100,000 of Germany's 550,000 to 600,000 Jews served for Germany in the war, and between 78-80% were front line soldiers. Von Hohenborn never published his findings, but the numbers managed to leak.
So the Jews were considered by nationalist Germans to be one in the same with the revolutionaries and Bolsheviks (an idea not specifically German, either. Was incredibly popular among much of the western world, not just the far right). As the train of thought follows "War lost by striking and rebelling civilians -> Working civilians riled up by Bolsheviks and socialists -> War really lost because of lefties -> Jews must be red, it's in their character and blood (thanks, Churchill) -> Jews are to blame for the loss"
There are some misconceptions here. Understanding German fascism needs more than a fast glimpse at WW1. Antisemitism in Germany was very strong since the 19th century. While the ruling aristocrats were less antisemitic than ever as a result of the age of enlightenment, antisemitism grew in the bourgeoisie as a result of a growing nationalism and a misconception of Darwin's theories. Basically these misconceptions about Darwin's theories were common in all of Europe and the USA. The results were for example enforced castrations of poor or mental disabled people.
When in 1848 the bourgeoisie together with the working class tried to establish a united Germany in a revolution against the small kingdoms om German territories, it was on the surface about democracy. But when in 1871 Germany was founded by Preussen, the call for democracy silenced, but not for the working class. The bourgeoisie silenced because the new country offered a much better economic base with free markets in Germany.
Many intellectuals of the working class were educated by clubs for working class education founded by liberal (European expression) members of the bourgeoisie. The was one of the reasons for the existence of a lot of Marxists who demanded participation instead of a revolution and were organized in the SPD . This was the foundation of the later success of "reformers" like Eduard Bernstein towards a political party well integrated into the political system of the Emperor. But the working class was since the foundation of unions (~1830) an enemy of the state because demands for a better living would result into lesser profits and the competition with the UK would be lose momentum. The workers movement was progressive and conservative a the same time. Most of them liked to show there are as good patriots like the members of the bourgeoisie, which lead to a nationalistic thinking in the large parts of the SPD against all Marxist theories and the same time they founded cultural new institutions like the Freikörperkultur (Movement for naked sports) , public theater and music from workers on a high level, which attracted a lot of intellectuals and many were "Jewish" (It's a problematic word based on wrong theories.) . Antisemitic haters had now 2 fronts: The Jewish world of business and the workers movement. When the Russian revolution in 1918 was won, it was just another ingredient for their hate, when leading Russian and Jewish intellectuals were supporting the revolution.
The SPD supported the war in 1914 by a direct order from the headquarter in Berlin. Opposing SPD politicians were excluded and in at least to cases in 1918 executed by a militia which was acting on the behalf of the new SPD government. Since the war created huge problems, the Army overtook the control of the country actual in 1916. The support from SPD members for the war vanished and a Marxists group the Spartakusgruppe grew stronger opposing the war. The leadership of the SPD tried in 1916 to silence opposing members which resulted into a split of the party in 1917. The resistance of leftists in WW1 was the background for the myth created by General Hindenburg and General Ludendorf that the leftists stabbed the army in the back at home. But the industrial efficiency was better than ever like in WW2. Nevertheless after the revolution in November 1918 which lead to the democratic Republic of Weimar this myth became popular between monarchists and right wingers. After 1918 the reparations were indirect partially paid with printed money which lead to the hyperinflation of 1923.
Hitler himself searched for the answer for a failing nation based on theories from the 19th century and large parts of the working class liked those theories as well. Even the leftist Kautsky justified colonies in Afrika with superiority. This word nation demands a focus on nationalism based on why the base of a German population is so bad resulting into loosing a war. It's not a question for individualism, but it includes the prerequisite of a racial problem of the population. To point onto the central idea: The definition of race wasn't consequent nor had a scientific foundation. Races are founding nations to be able to live and getting stronger by forming a nation. So what is wrong with the German population? He searched for parts of the population which aren't nationalists (Marxists) and aren't "German" and he found Jews. Because Jews have survived without a nation the conclusion was, they must be parasites. It's written in "Mein Kampf". It's a boring book, but for historical reasons interesting. This hole idea to kill Jews because of the failure of a nation is freaking banal, but in mind of a nationalist rational. The roots of such a thinking are grown since utilitarians like Bentham and since the french revolution with the invention of patriotism.
Just to preface this, although Hitler had these feelings, it was not these feeling alone that resulted in his policies towards the Jewish people and eventually the Holocaust. If you read his writings, he speaks more often of Jewish people and Communists in the East, and their threat, than blaming them for Germany's defeat in WW1.
So here's a bit on one big reason Hitler went after Jewish people, in general:
In addition to what others have posted, according to some historians, Hitler was influenced by a group called Aufbau Vereinigung (and similar, less official groups). I believe there is even at least one documentation of him attending a meeting.
Anyways, on the grand chess board that nations play on, the real threat coming out of World War 1 was not a future war hungry Germany but rather Communism. It was as much a threat to the West then as it was after World War 2 in that it scared the shit out of those in power - from ceremonial positions such as the King of England to industrialists. Imagine you're an oil, steel, banking or car magnate in America - you just became wealthier than anyone in history at the turn of the century and now some strange power wants to take it all away from you.
So taking a step back - a lot of people don't realize this - but communism in Russia was such a threat to the West that British and American troops fought in the Russian Civil War. We were aligned with what was then known as the White Russians and fought until around 1919.
After that point other pro-White Russian troops and groups were driven or emigrated from Russia into Germany, Poland, and other surrounding countries. They brought their horror stories, propaganda and began spreading these stories with support of Western backers.
One such group was Aufbau Vereinigung, which met around Southern Germany and supposedly was indirectly backed by Henry Ford. Groups such as this used Anti-semitism as part of their attack on communism and as a way to link it back to Germany. If Jewish people were part of the international conspiracy for Communism and they also lived in Germany, then there were enemies already within the motherland. This played into other ideas and events that happened during WW1 - because often Jewish people were labor organizers. During the war there were strikes at armament factories and Hitler and others linked these to organized labor... organized by the Jews. And so in the mind of people like Henry Ford, Hitler and others, organized labor is one big slippery slope to Communism.
Some historians argue that the threat of Communism is why so many Western leaders looked the other way when Hitler came to power, armed Germany and went on the war path. Especially because if you read his early writings, Hitler wasn't interested in taking over places such as Britain, but instead taking over Russia and enslaving the native, inferrer, people so that they could support the Third Reich.
Edit: should also add note about reparations and those that met at Versailles on behalf of Germany were also a huge thing
I can take a crack at this (sources from Mein Kampf, Ford Translation).
Hitler believed WWI was lost due to propaganda, i.e., Germans were subject to enemy and "Jewish" propaganda including newspapers, causing them to lose faith in their soldiers, government, and people and ultimately lose the war. This was unacceptable to Hitler and is one of the reasons the Nazi party used so much propaganda.
Hitler believed this during and after WWI (at which point he was blind and possessionless) but didn't begin his public antisemitism until mainly after the war when many parties (including the Nazi party) began to rise with leaders who did not fight for the war as soldiers and were mostly Jewish (or "deceived" by the Jews). This, coupled with his observation that almost all newspapers at the time were written by Jewish authors, caused him to believe that it was the Jewish people who were responsible for the propaganda that caused the loss of WWI.
Three things I'd like to point out: This explanation is how Hitler began to ask The Jewish Question. His complete hatred for the Jewish people is based on much more than these observations. I also cannot comment on the actual statistics of how many of these people were actually Jewish.
For a better explanation, I'd suggest picking up your own copy of Mein Kampf, which goes into much more detain than I can in a Reddit comment.