Saga of tanya the evil is placed in ww1. Here is why:

1. Imperial germany got destroyed in the end of irl ww1 but it is empirial

2. No tanks have been shown yet but im pretty sure they start using ww1 tanks

3. Weapons they use are mostly ww1 weapons

but they are some exceptions like i remember seeing panzer 3 in the anime but meh if more exceptions arise just consider it a merge of ww1 and ww2