Of course the commander is pretty sure you are Dorinthians, and your accents will likely give it away, although you may attempt to bluff them and say you are from an unaligned village that is along the border of what was Dorinth.
As for their gripe, Dorinth was just crushed and enslaved by the Tallione-Cassus empire (Cassus being their weaker neighbors incorporated into the empire). Originally I had a Russian sounding name for the Imperials, but I decided to change it for a more Roman sound after the big crash.
I always give my players a way to get out of tough spots... and I'll just throw in that the party notices that the Imperial soldier you are facing has a slight Dorinthian appearance to him... perhaps from a not-too-distant relative.