@Glomoro
If I'm not mistaken, it was also mentioned at the start that they held an aggressive expansionist military doctrine. Also, if they're a new country in the middle of a bunch of old, powerful ones, didn't they take land to get as big as they are? There were clearly wars before this one, either in whatever came before the Empire, or before the story starts. Or both, even.
That's not even referring to the fact that they're clearly the German Empire, which only came about due to beating down France and several others in the very late 1800s, unifying under Prussia. That can be considered pointless simply because this is a different world's timeline, clearly altered, the defense pacts and war itself are different, etc. But it does model itself after our own Imperial Germany.
Here in this world, the Dacians sent their 600,000 target idiots under the pretense of taking back their lands. Which means at some point, something was taken from them, even if only on paper in the way that you can say they were seized by combat.
But it's that doctrine that I'm fairly sure was mentioned early on that's the problem. If the new kid on the block shows up and is surprisingly beefy, sitting around talking about how he's going to start taking everyone's stuff after he beats the crap out of them, then that's a very good reason to kneecap him, isn't it? That expansionist view can only mean, "We are here to make war and take what is yours." It's practically the definition.