They thought they were getting cheap labor, and possibly inviting a little "hybrid vigor." Instead they picked up a nasty, virulent parasite.
But only some of them have realized that. Merkel... not at all.
Humans originated in Olduvai gorge in East Africa and invaded every continent. Europe has been invaded for centuries by Africans and Arabs and Mongols. Nothing new.
There was a German politician, I forget who, who said “We thought we were getting workers and instead we got people.”