Uhhuh. The empire's soldiers attacked first. You just didn't notice it. The prince gave the order. The emperor, being the cold dude he is, didn't bother to overrule it because a few soldiers can always be replaced. He probably also wanted to see the power of Japanese for himself. I guess you would have been happier if the main character had died here because he wouldn't have had the guts to protect himself and his troops. Great thinking, man.

The Japanese aren't here negotiating a peace between two equals. They are there to dictate terms and then play some negotiation theatre to make the terms more acceptable. They said that aloud in the previous episode. The other choice would be to slaughter the empire's population, but that wouldn't sit too well with the Japanese voters. So, they would rather have peace. The empire attacked first, Japan won, so the justice is doubly on the Japanese side (since the winner of a war is always right by international standards that most countries in Europe and elsewhere know all too well from history). Things got even worse for the empire when the existence of the Japanese slaves was revealed. The empire suddenly had all the more to pay for.

Kind of strange a German wouldn't get this considering how the USA and USSR split your country after the WW2 and did what the hell they wanted with their shares, especially the Soviets.