Answer: pure pragmatism. Anti-semitism was quite common and spread in all the Europe already in the 19th century. There were numerous anti-semite organizations in many European countries. In Germany after WW I defeat German extreme right accused Jews from the defeat (Jews were usually made responsible for the "decadence" phenomenon) and considered <em>Weimar Republic </em>as Jewish. This was quite common and usual opinion between Germans at that time. Hitler was aware of that so to get votes and popularity Hitler started preaching not only nationalism and socialism but also antisemitism. Like that he was able to win legally elections for the position of<em> Kanzler</em> (he took this position in January 1933).
Explanation: Hitler initiated very brutal antisemitic politics so he became soon very popular. He explained his strategy a long time before 1933 in his book <em>Mein Kampf</em> (My Struggle).
Hitler believed the Jews caused all of Germany's problems after World War I. The reason being is that Germany was absolutely decimated after World War I, The United States and it's allies imposing very harsh penalties and sanctions on them. Hitler needed to boost the morale of his people and army, to fight back in what would soon become World War II.
The crusades were launched by Christian Europe to take away control of Jerusalem and the Holy Land from Muslim hands. Muslims never conquered two-thirds of Christian lands, and they never defeated the whole Christianity.
