What if it had been a German soldier that Diana had found on the beach and he had found plans by the British to destroy Berlin with a deadly weapon? It would have been a bold move.
But how would the movie have played to international audiences, and specifically to English speaking audiences, such as the United States and United Kingdom?
It would have been a more counter-intuitive choice, but I don't think it would have worked or been a good idea.
As I understand it, historians differ on who was mostly responsible for starting the great war, but Germany being primarily responsible has been the dominant thesis for a long time, or Germany and Austria-Hungary. That idea has been called into question more recently, with other ideas being proposed, such as more of a shared responsibility, and so on. (Though I think the Germany/Austria-Hungary idea is still quite widespread.)
That lines up with audience expectations for the most part, exacerbated by WWII movies, in which the Germans are the bad guy.
Undermining those expectations is the whole idea of the movie's main arc: Diana starts with a clear expectation of who the bad guy is, which lines up with audience expectations, but that idea evaporates gradually and we end up with the concept of shared responsibility. All sides in the war are partially responsible.
If the movie started with the counter-intuitive idea, that sort of shared journey on the part of the main character and the audience would not have been possible.
OR would give way more weight to Ares coercing her.
In what sense? Ares doesn't argue that the Germans are evil, but rather that all sides in the conflict are equally worthy of contempt. I don't think the question of who is bombing whom would really have changed his argument at all.