When you forget which world war you're fighting in

Prussia still existed in ww1

It just says German Empire next to it

Silvermyth is right, I think, it's not the right flag. The German Empire refers to a period of time after the German Unification, which included Prussia. The German Empire collapsed in 1918, alongside the end of WWI, so it would be accurate for the game to use it. However, the flag here is depicting the Prussian Eagle in front of an iron cross. While both of these symbols were used a helluva lot, the flag itself is a simple tricolor of black, white, and red.