Prussia just compared Arthur to England so, to the dragon, that looks like Prussia doesn't want anything to do with him. Makes things a helluva lot simpler, really. Why couldn't they continue doing that?

"Then you don't whine about me being a pompous know-it-all!" He snaps his teeth at Prussia, tone bitter. "I don't deserve to be called names, or poked, or told my breath smells!" He didn't deserve to be teased, either. All Arthur wanted to know was if Gilbert survived!