I was curious as to whether in history there has ever been a massive demographic shift in Western Europe coming from the Near East or Northern Africa. From my very limited info, I don't believe this has ever happened before. At current rates, Middle Easterners/North Africans are approaching 10% in many Western European countries.
The countries in Western Europe I am primarily referring to are:
Can't speak for historic times, but in prehistoric times there was a massive influx.
Farming was invented in the middle east, and between 6,000 - 9,000 years ago there was a large scale movement of farmers from the Middle East to Europe. By "large scale" I mean that even today most Europeans have about 40% genetic contribution from the Middle East.
In some parts of Europe, Middle Eastern farmers completely displaced local populations. In other places, local populations continued as marginalized hunter gatherers until they were absorbed by the farmers.
This process could take a long time. Genetic evidence from Sweden shows that Middle Eastern farmers dominated the southern farmland, pushing native hunter gatherers further north. They remained genetically separate for over a thousand years before they finally intermingled and produced the modern Swedish population about 5,000 years ago.
This is all prehistoric stuff so it doesn't answer your question about what social changes happened (agriculture was introduced) or how they assimilated. But it might interest you to know that almost half of European ancestry is Middle Eastern.
As an addendum to this question, I presume that one of the likely instances would have been the Moorish population in Al-Andalus. Now obviously there was a huge practicing Muslim population on the peninsula at the time, but seperate from that do we have any idea of what % of the population in Spain by say 1000AD or after would have been ethnically from North Africa?