It's a well charted out plan.
European political elites in the 1960s tell the citizens of their nations of guest worker policies, where imported workers will handle manual work, while welfare will be used to support the nation's poor so they can rise to higher socio-economic status.
Governments use contracted labour from Third World countries to do construction work and the building of large public sector projects, with the "guest worker" policy having an appropriately misleading euphemism for people to think that as "guests", those workers are there on temporary basis. General xenophobia among the common man would not allow it to be accepted otherwise.
With government contracted labour being brought in, hundreds of people from foreign nations would be given easy visas, and simply dumped in large numbers into different neighbourhoods or in government-sponsored housing.
As large number of people who don't even speak the local language are virtually dumped onto the country, the wages for such low-end jobs keep decreasing. Eventually, the children of these immigrants grow up in an environment with even less jobs available for them. They grow furious, all the more because they were forced to be born in a country where their own parents don't understand the culture. Crime by immigrants goes up on a huge scale by the 1980s, with Algerians in France declaring themselves to be in civil war with the natives.
Although European political elite try to appease natives by ensuring they integrate, their policies have gone so far, that it's difficult to integrate the giant amount of foreigners they brought in, just to get the votes that put them in power. Hence, they turn their efforts in public schools and public sector workplaces to get their native populations to integrate to foreigners.
Police are instructed to take complaints against immigrants with a grain of salt. Magistrates are instructed to overlook certain felonies in case of certain cultures. Schools are told to indoctrinate children to live according to foreign cultures, habits, and outlooks. Any minor complaint against an immigrant is to be seen as an act of discrimination, and should also be punished by law enforcement just to discourage it from happening. In Britain, heads of police departments ignore death threats made by Muslims who would later do the London metro bombings. In France, Algerians and Morrocans are to be released or given light sentences by police for public vandalism. Multiculturalism and diversity thought control becomes new public policy.
Xenophobic political parties come up. They are successful in getting the support of a significant portion of population. Other political parties simply label them as Nazis, and as a part of their new plans to remain in power, offer more concessions to their new foreign natives to ensure to get their votes.
If a free market flow of labour ever actually happened, people would take the personal cost of learning foreign cultures and foreign habits, instead of having to impose that cost on the rest of society. They would ensure to go to those nations which offer the highest wages, and by comparing it with their own costs of resettlement, instead of being given an easy pass and an early headstart by state-sponsored contractors, only to end up in places where work is saturated. They would have located themselves and placed themselves in those nations where they would be happy for the most consistently long period, instead of being able to come into a country, being absolutely confused, and having the local governments integrate their own people according to these foreigners. And most of all, natural xenophobia would be challenged as little as possible, as immigration would start from among the higher end of the social ladder, and among the societal elites who are more receptive to it, than from the bottom of the ladder.