This answer may not satisfy the subReddit rule calling for “academic-level answers”, but I’ll have a shot answering you anyway.

I think you’re asking for a potted 2000 year history…

The starting point must be with the Jewish communities of the Roman Empire. In 135 CE the Empire, fed up with Jewish resistance to Roman rule in Judea (roughly modern day Israel/Palestine), brutally punished the territory for its latest revolt. Thousands of Jews were killed, towns and villages across the province were destroyed, Jews were banned from living in Jerusalem and the province itself was renamed Syria Palaestina. Although Jewish communities remained in the province, the bulk of the surviving population was scattered across the Empire. https://en.wikipedia.org/wiki/Bar_Kokhba_revolt#Immediate_consequences

After the fall of the Roman Empire (in the 5th century CE) Jewish communities survived in various places, but were always in a minority and potentially subject to persecution as such. Not surprisingly, the places where they were NOT persecuted were the places where they were able to develop thriving communities. For example, in the period from the second half of the 8th to the 11th century Jews received favourable treatment under the Islamic rulers of Iberia and Jewish communities there were large and successful. The Jews of Iberia came to be called the Sephardim (literally: “of Spain”). http://www.sephardicstudies.org/islam.html

In central Europe between the 4th and 9th centuries, Jewish communities spread east into the German-speaking lands of what became the Holy Roman Empire. In this period Judaism does not seem to have been particularly an object of persecution. Jews were artisans and farmers, as much as merchants. This is the period when Yiddish developed. Jews of Germany came to be known as the Ashkenazim (literally: “of Germany”) All this changed in 1095 with the proclamation of the First Crusade, which not only focused European Christianity on war with Islam but also put a focus on Jews as outsiders. http://www.jewishvirtuallibrary.org/jsource/vjw/germany.html

But even as Christian persecutions of Jews became increasingly serious in western Europe and Germany, further east a new haven opened up. From the founding of the Kingdom of Poland in 1025 through to the early years of the Polish-Lithuanian Commonwealth created in 1569, Poland was the most tolerant country in Europe – not just for Jews. In the Polish Kingdom the population included western Christians (Catholics), eastern Christians (Orthodox), Pagan Lithuanians (at least for a period) and even Moslem Tartars as well as Jews. All these religions existed if not in harmony at least in a general atmosphere of toleration for much of the time. This was legalised in 1573 when the signatories of the Warsaw Confederation pledged never to impose a faith by force. http://scholar.harvard.edu/pmalysz/content/heretics-house-polish-tolerance See also: https://en.wikipedia.org/wiki/Demographic_history_of_Poland

From the 10th century the Jewish communities across the rest of Christian Europe, even where they were not persecuted or banned, were subject to increased restrictions. Most significantly, Jews were forbidden from owning land or farming. For the whole of this period Europe was a subsistence economy – meaning almost everyone worked the land. If you couldn’t work the land, you had to live in a town and you had to work with trade. From 1500, only in Poland were Jews allowed to farm. In connection with this, it is interesting to read the clauses of the Magna Carta to see how Jews were viewed in England in 1215. http://www.thejc.com/comment-and-debate/essays/122219/magna-cartas-three-jewish-clauses

Jewish communities were also expelled – from Italy in 855, from France in 1182, from England in 1290, from Hungary in 1349, from Spain in 1492, from Portugal in 1496. These are a small selection as Jews were often allowed to return only to be expelled again. http://biblebelievers.org.au/expelled.htm

However, for a period of nearly 800 years, the Jewish population of Poland was able to grow largely unchecked. Even after Greater Poland was divided up among its neighbours between 1772 and 1795, the Jewish population in the areas that had been Poland remained relatively large. Large enough so that when Poland was re-established after the 1st World War the new state had the largest Jewish population of any European Country. http://www.jewishvirtuallibrary.org/jsource/vjw/Poland.html And again see: https://en.wikipedia.org/wiki/Demographic_history_of_Poland