Except an influx of criminals of all kinds (gangsters, thieves, robbers, killers, sex predators) IS a threat to national security. We have enough of our own, we don't need to import more.
Also, if you allow an entire bloc to sustain the fires of nationalism contrary to the identity of the nation in which they ACTUALLY reside, you WILL end up with a security threat, genius.
I'm pretty sure that North African and Middle Eastern guest workers in Europe didn't cause many problems when they first showed up. The first generation of them was probably pretty cool.
I agree we need more assimilation and less criminals : )