Thanks to history.com for this retelling of those eerie foo fighters during World War II … my takeaway: They stopped appearing once the German army was defeated:

https://www.history.com/news/wwii-ufos-allied-airmen-orange-lights-foo-fighters

One hypothesis I favor: The Pentagon grabbed the technology along with the German brains behind it, later perfecting various vessels based on the breakthrough propulsion system.

But I’m not telling you anything you don’t already know! It’s all in Flying Saucers.