I read in a novel once that Imperial Russia had plans to slowly infiltrate the USA by way of migrating Siberians coming in over the Bering Strait. The idea would be that they would gradually move down into central USA and set up shop. Over generations they would gradually outstrip the locals and become the most prominent ethnic group in the US, a giant sleeper cell. I can't be sure how much of this is based on fact but I think the roots of it probably were.
One can also argue that the US is slowly being invaded by large numbers of Latin peoples, for instance via Mexico and Cuba by way of Miami. Indeed, if you go to Miami these days it's hard to find anyone speaking English.