Islamic world. This event forms the framework for Yehuda Halevi's work The Kuzari (c.1140), but how much the traces of Judaism within this group survived the collapse of the Khazar empire is a matter of scholarly debate. Arthur Koestler, in his book The Thirteenth Tribe (1976), and more recently Shlomo Sand in his book The Invention of the Jewish People (2008) have recently (controversially) theorized that East-European Jews are more ethnically Khazar than they are Semitic. However, numerous genetic studies have not supported this theory. In western Europe, following the collapse of the Western Roman Empire in 476, and especially after the re-orientation of trade caused by the Moorish conquest of Iberia in the early 8th century, communications between the Jewish communities in northern parts of the former western empire became sporadic. At the same time, rule under Islam, even with dhimmi status, resulted in freer trade and communications within the Muslim world, and the communities in Iberia remained in frequent contact with Jewry in North Africa and the Middle East, but communities further afield, in central and south Asia and central Africa, remained more isolated, and continued to develop their own unique traditions. For the Sephardim in Spain, it resulted in a "Hebrew Golden Age" in the 10th to 12th centuries. The 1492 expulsion from Spain by the Catholic Monarchs however, made the Sephardic Jews hide and disperse to France, Italy, England, the Netherlands, Scandinavia, parts of what is now northwestern Germany, and to other existing communities in Christian Europe, as well as to those within the Ottoman Empire, to the Maghreb in North Africa and smaller numbers to other areas of the Middle East, and eventually to the Americas in the early 17th century. In northern and Christian Europe during this period, financial competition developed between the authority of the Pope in Rome and nascent states and empires. In western Europe, the conditions for Jewry differed between the communities within the various countries and over time, depending on background conditions. With both pull and push factors operating, Ashkenazi emigration to the Americas would increase in the early 18th century with German-speaking Ashkenazi Jews, and end with a tidal wave between 1880 and the early 20th century with Yiddish-speaking Ashkenazim, as conditions in the east deteriorated under the failing Russian Empire. After the Holocaust, which resulted in the murder of more than 6 million Jews living in Europe, North