Okay, so, i either had the dumbest, and most Pain in the ass attempt at trying to maintain logic and cohearance in a nonsensical alternate universe.
Or the Idea to finally make me a millionare so here we go:
1800ish Napoleonic Warfare, but in a WW1 fashion.

So Imagine The Napoleonic Wars, with Flintlocks, Cavalry, Coordinated fire, Musicians, and overall pre revolution france behaivior.

but instead of glorious European line battles, you throw some trenches, artillery, heavily armored vehicles, 90% of all Countries (Even the pacifist ones), and of course Mustard Gas.

So this way you’d have Inhumane and Traumatic battles, Beasts of Steel and flesh, Heavy experimentation eras, alongside nightmarish living conditons, in a time where War was still done with manneurisms.

As to why they developed Vehicles and New tactics, yet the firearms stayed being Blackpowder weapons…
I don’t know, read the first sentence again.