The EU or the European Union is a political and economic union of several European countries (currently 28 member-states), that includes most of the jusridictions on the continent. The EU has developed in is actively promoting a range of policies aimed towards bringing the member-states together in all respects, as well as making their cooperation easier. These policies include:

common internal market
standardised system of laws
non-restrictive travel area
common visa policy
free circulation of goods and services
free movement of labour

Additionally, these benefits can also be enjoyed by the family members of said immigrants - the EU has many policies that allow family members to obtain residence permits easier, involving fewer documents and employing simplified procedures.

History
The history of migration in the modern European Union begins with the end of the World War II, and is based on the idea of a unified, borderless Europe. However, this idea or, more loosely, an idea of a European unity, has been present even before that.

Before the end of World War II
The ideas of a united Europe before the 20th century (1949, when the Council of Europe was established, in particular) have been expressed by dynastic unions, empires and country-level unions, who often presented themselves as the "heart of Europe", the ones who embodied the political essence of the continent. The notable examples of such entities would be the Holy Roman Empire, the Polish-Lithuaniuan Commonwealth and the Austro-Hungarian Empire.

the painting named queen europeNotably, some thinkers have also expressed the idea of Europe being united not under the banner of a specific country, kingdom, etc., but as a sovereign power in itself. Thus, for example, a German cartographer Sebastian Münster in his 1570 work "Cosmographia" presented a painting named "Europa regina" or "Queen Europe". In this painting, the countries of Europe are depicted borderless and united into a single entity, whose borders roughly corresponded to the borders of Europe the continent.