WITHOUT employing Extraterrestrial Interstellar Chiropterae, and with a POD after the OTL Civil War, bring about a Fallout/Max Max-esque scenario with America getting 'stuck' in the style/tech/etc. of the Gilded Age (i.e. 'steam/coalpunk', or pre-diesel) instead of the 1950s post-war period.
Basically, turn ALL of 1880s/90s America into the wildest of wests.
No nukes!