@Dyoneva
If we were to make an analogy using WW1 to explain why everyone wants to attack the empire, it is because of the alliances between the countries, for example, Austria-Hungary vs Serbia would have been justified if the attackers were from the serbian army, but that wasn't the case, either way, AH declared the war on Serbia with the justification of the assasination of our friend Franz (the true father of anime), however, Germany stepped in, being allied with AH, and since Serbia was friends with other countries, they also intervened. In Youjo Senki, the Entente was this version of Serbia, however, they attacked directly and the Empire invaded them in response, but the other countries (notFrance and notUK) didn't recognize said invasion, thus justifying their reason to attack the empire, from there, the rest of countries have their own motivations to also jump in against the Empire (mostly territory and resources).