Map from the Jewish Encyclopaedia showing
the distribution of religions in Europe during the Middle Ages
An interesting matter related to this conspiracy theory is finding out who where these European Jews, why did they settle throughout Europe, and most important, why did they excel in finances. To answer these questions we need to go back to the times of the Roman Empire and follow the Jewish Diasporas.
After the First Jewish–Roman War in Judea (also known as the Great Revolt), the destruction of the Temple of Jerusalem in 70 CE, and the Bar Kochba rebellion of 132-135 CE, the Romans no longer recognized the authority of any Jewish body, and the Jews were prohibited from living in Jerusalem.  Furthermore, after Christianity became the official religion of Rome and Constantinople in 380, Jews were increasingly marginalized, and brutally persecuted. The Jewish people spread throughout the Roman Empire into three groups and developed their own distinctive traditions. These three groups were: the Mizrahi Jews, the Sephardi Jews and the Ashkenazi Jews (or Ashkenazim).
The main occupation of the Jews that remained in Palestine and Mesopotamia was still farming. However, in the Diaspora communities trade was more common. Talmudic learning began to emerge and assert its authority over Jewish life throughout the Diaspora, and the Rabbinic Judaism created a religious mandate for literacy, requiring all Jewish males to learn Hebrew and read from the Torah. This emphasis on literacy and learning a second language would eventually be of great benefit to the Jews, allowing them to take on commercial and financial roles within Gentile societies where literacy was often quite low and education reserved for the clergy and the nobility.
The Mizrahi Jews settled in the Middle East and North Africa, and after the Islamic conquest of these regions, trade and commerce allowed them, as a highly literate people, to abandon farming and engage in occupations where they could better use their skills.  In the Caliphate of Baghdad, the center of the Jewish world, the Jews took on many of the financial occupations available.
The Sephardi Jews settled in the Iberian Peninsula (today Spain and Portugal) during the Roman occupation. As citizens of the Roman Empire, the Jews of Hispania engaged in a variety of occupations, and until the adoption of Christianity, Jews had close relations with non-Jewish populations and played an active role in the social and economic life. However, the Christian authorities, concerned with the Jewish presence, began a series of prohibitions, like: marriage of Christians with Jews, blessing of Christian crops by Jews, and sharing of meals by Christians and Jews. The situation completely changed after 587 AD, with aggressive policies against them, which culminated in many orders of expulsion, forced conversion, isolation, enslavement, execution, and other punitive measures.
The third group, and the one which our conspiracy theory is most interested in, were the Ashkenazi Jews; which settled along Northern and Eastern Europe. At some point in the late 8th or early 9th century, the Khazar royalty and nobility converted to Judaism, and part of the general population followed.  The first Jewish Khazar king was Bulan, followed by king Obadiah, who strengthened Judaism, invited rabbis into the kingdom and built synagogues. The descendents of these Jews moved to the Rhine in Germany, from Alsace in the south to the Rhineland in the north; in the region of “Ashkenaz”, which in medieval Hebrew means "Germany". There, they also took many of the financial occupations, including money lending (or usury).
Some modern researchers doubt that these Ashkenazi Jews are from a Jewish origin at all, and affirm that they are the descendents of the Khazars that converted to Judaism during the 8th and 9th century. However, DNA studies carried out along the 1990s cleared up this issue and offered an explanation as to why many Ashkenazi Jews have European and not Middle Eastern features (e.g. blond hair and blue eyes). In regards to the Y chromosome (passed on only by the father), a study published in 2000 showed that 12.5% of their total admixture of the Ashkenazi Jews contains mutations common amongst Middle Eastern peoples.  And another study from 2005, based on Y chromosome polymorphic markers, showed that Ashkenazi Jews are more closely related to other Jewish and Middle Eastern groups than to their host populations in Europe; though 11.5% of male Ashkenazi were found to belong to R-M17, the dominant Y chromosome haplogroup in Eastern Europeans.  On the other hand, another study published in 2001, showed a 12.7% frequency of the Eu 19 chromosomes (common in Eastern European peoples).  In regards to the Mitochondrial DNA (passed on only by the mother), a study from 2006 based on high-resolution analysis of haplogroup K(mtDNA), suggested that about 40% of the current Ashkenazi population is descended matrilineally from just four founder lineages, that were likely from a Hebrew/Levantine mtDNA pool originating in the Middle East in the first and second centuries CE. In addition, the rest of Ashkenazi mtDNA originates from around 150 lineages, most of those likely of Middle Eastern origin.  These studies clearly show that the Ashkenazi Jews are in fact a mixture between Middle Eastern and East Europeans races, and confirms the theory that proposes intermarriages between Jews from the Middle East and Khazars between the second and ninth centuries CE.
… Next Chapter: The Rothschild Family
Wikipedia: Ashkenazi Jews (http://en.wikipedia.org/wiki/Ashkenazi_Jews)
Wikipedia: Sephardi Jews (http://en.wikipedia.org/wiki/Sephardi_Jews)
 Seth Schwartz, "Imperialism and Jewish Society, 200 BCE to 640 CE”, pp. 103–128, Princeton University Press, 2001.
 Maristella Botticini and Zvi Eckstein, "From Farmers to Merchants, Voluntary Conversions and Diaspora: A Human Capital Interpretation of Jewish History”, Centre for Economic Policy Research, 2006.
 Peter B. Golden, ”Khazar studies: an historico-philological inquiry into the origins of the Khazars”, Akadémiai Kiadó, Budapest, 1980.
 M. F. Hammer; A. J. Redd, E. T. Wood, M. R. Bonner, H. Jarjanazi, T. Karafet, S. Santachiara-Benerecetti, A. Oppenheim, M. A. Jobling, T. Jenkins, H. Ostrer and B. Bonné-Tamir, "Jewish and Middle Eastern non-Jewish populations share a common pool of Y-chromosome biallelic haplotypes", Proceedings of the National Academy of Sciences 97, 2000.
 Almut Nebel, Dvora Filon, Marina Faerman, Himla Soodyall and Ariella Oppenheim, "Y chromosome evidence for a founder effect in Ashkenazi Jews", European Journal of Human Genetics, 2004.
 Almut Nebel, Dvora Filon, Bernd Brinkmann, Partha P. Majumder, Marina Faerman, Ariella Oppenheim. "The Y Chromosome Pool of Jews as Part of the Genetic Landscape of the Middle East", The American Journal of Human Genetics (pp. 1095–112, Volume 69, number 5), 2000.
 Behar, M. Doron; Ene Metspalu, Toomas Kivisild, Alessandro Achilli, Yarin Hadid, Shay Tzur, Luisa Pereira, Antonio Amorim, Lluı's Quintana-Murci, Kari Majamaa, Corinna Herrnstadt, Neil Howell, Oleg Balanovsky, Ildus Kutuev, Andrey Pshenichnov, David Gurwitz, Batsheva Bonne-Tamir, Antonio Torroni, Richard Villems, and Karl Skorecki, "The Matrilineal Ancestry of Ashkenazi Jewry: Portrait of a Recent Founder Event", The American Journal of Human Genetics 78, 2006.