In the late 1950s a number of European countries (most notably West Germany and France) decided on a migration policy known as the buffer theory.
Owing to rapid economic recovery in the post-World War II period (aided by the American Marshall plan) there were many more job vacancies than people who were available or becoming available in the workforce to fill them.
To resolve this situation the countries decided to "import" workers from the southern Mediterranean basin (including North Africa) on a temporary capacity to fill this labour shortfall.
These workers were invitees of the governments and came to Europe initially on the understanding that they could at any point in time in the future be repatriated if and when economic circumstances changed.
This created a difficult situation for the German government which became increasingly worse as the number of immigrants swelled to their highest levels ever.