From a reader:
In your post, you suggest that the current immigration pattern (large numbers from one country settling on the border, not assimilating) has never happened in the US before.
This is true in the strictest sense, but if you include future US territories, then it happened in Texas when it was still part of Mexico. The Mexican government encouraged European settlement in their largely unpopulated northern territories (taking “homesteads Mexicans won’t take?). The newcomers never assimilated, grew frustrated at their isolation from both their legal government and their old culture, and eventually used the power of demographics to force a revolution.
Anyone who thinks Mexicans in the US don’t remember this hasn’t talked to very many of them.