This argument never made sense to me. Why would the rest of the world being poor cause a huge middle class in America? Why would the rest of the world recovering cause the US to suddenly get poorer.

Like post post ww2 say we produced 1 car for every American. Also we produced 1 house for every American. Every car and house was produced in America because Europe was bombed to shit. Now 20 years later, Europe has recovered a bit and can start producing cars and houses again. Why wouldn't the US still be able to produce 1 car for every adult? Oh sorry, Germany is no longer a pile of rubble, you and your spouse need to share a car now. Also your adult kids need to move back in with you, no house for them either.

This is obviously absurd. US would be even richer since they no longer had to spend massive amounts of money funding the war effort and then massive amounts of money rebuilding Europe. Hollowing out the US middle class was a choice, not some law of nature.

After WW2, Europe and Asia were rubble, and needed to rebuild. And the systems, structures, and customs that had existed pre-war had fallen apart. They all needed, simultaneously, to rebuild and modernize.

To do that, they needed cars, machinery, home goods, electronics, etc. They had the labor to produce those things, but not the infrastructure. It takes time to build factories, and a skilled labor pool, and a logistics network, and so on.

So where did you go to get the goods & services you needed to rebuild? There was really only one option. The US was exporting cars, factory equipment, heavy machinery, steel, radio, coca cola, etc. They had an intact industrial plant, and had lost (relatively speaking) very few working-age men in the war. That helped them ramp up quickly with internal demand (fed by pent-up war wages).

For reasons laid out above, it wasn't practical to move factories overseas, or outsource parts, or automate. So workers in the areas with factories were in very high demand, and wages went way, way up in those areas. That had knock-on effects: America was just beginning to import oil in large quantities, so American coal & oil was suddenly in high demand. Same with mining, logging, etc. That caused a general boom--specifically favoring labor.

It wasn't because the rest of the world was poor that the American middle class was rich. It was because the rest of the world was developing, and America had a near-monopoly on the means of doing it. What's happened in the meantime is just that the US has lost that monopoly. Now American workers face relatively fair competition. This has been a huge net positive for the world, with cheaper goods and higher wages pretty much across the board...except for American workers.