The major capitals in Europe have tried to integrate middle eastern and African (mostly Muslim) immigrants for decades, without success. These people seem to flock together into very tight knit communities based on religious and ethnic traditions. They shun western culture and education and have developed their own religious/ethnic biospheres within these large cities. As a result many can't get living wage jobs and are effectively wards of the social welfare state. IIRC the term "no-go zone" was first coined in France because many of these enclaves are openly hostile to any outsider presence or influence. As a result, the political/cultural tensions are so frayed, that in some cases not even police and fire departments venture in these zones unless they have permission from the local religious leadership.
You see that's it right there. People not wanting to assimilate. Try that in any other country in the world, and see how far you get.
I just love the liberal excuse, "We're a nation of immigrants" or "We're all immigrants". Another code word. Yes we are all the products of immigration - unless you're native American Indian (but even then they are descended from Mongolians who came over the land bridge between Russia and Alaska, so, technically, they're "immigrants" too!) - you're descended from immigrants. But the vast majority were LEGAL immigrants, and they came through a barrier (i.e. Ellis Island or something), and - when they came - they left their mother country behind and became immersed in their adopted home - America. Lots of people were turned away because of disease, lack of documentation, no sponsor or established family, etc. But they assimilated.
Nowadays alot of immigration is illegal, in the back door, they don't assimilate, and they don't leave their home country behind. they come to get all the bennies of the USA and don't want to become an American. Sure I get congregating with like ethnicity - like NYC's Little Italy - but you have to assimilate. America becomes your home. You get all the advantages and privileges - and you assume all the responsibilities and abide by the laws. Most of these illegals don't want that. They want the benefits with none of the responsibilities. Sorry, it's all or nothing!
But the legal pathway is too slow and takes too long and.....TOUGH! It's our country, and we make the rules. If you don't like the rules don't come here! Or, do it legally, go to law school, and be a part of changing those rules.
The only reason they get by is because of big business wanting cheap labor (so they can get richer) and politicians wanting votes. And since politicians control the purse strings, they permit only a Swiss-cheese approach to border enforcement, and we have what we have. A physical wall - once built - would be (for the Southern Border) a true impediment. Perfect? No. Stop everybody? No. But it would be a big help.