I've often wanted to research what caused the downfall of nations and empires to see if there are similarities and lessons to be learned to ensure America doesn't travel down the same path. Or perhaps nations falling is inevitable. There is ample evidence - Rome, Great Britain, Spain, Portugal, France, the Mongols, ancient Greece, the Soviet Union and many others all eventually fell by the wayside or simply faded from greatness.
Many people have looked into the decline of various empires. Here's a recent example worth watching (NOTE, it's preceded by an advertisement that you can skip if you like). There's not an in-depth dive here but the book seems like it might be worthwhile.