Thanks and I can see a bit of both sides on this one as you are not wrong on having to declare war on Belgium, yet since the historical DoW didn't happen until the 4th, that arguably works out well as it can allow players to entertain some alternate strategies (alt-history) if they want to hold off on declaring war on Belgium and focus on Russia first instead.
For Luxembourg, if I remember correctly, might have seen the first action from German soldiers in the entire war as I believe plans were for the Germans to invade on the 1st. While they didn't follow through, some units apparently didn't get the message and I believe took a town before retreating.
1) On the one hand the game starts on the 1st, arguably there was action in Luxembourg on the 1st (even if very limited), then the Germans did take it on the 2nd. For Russia, the Germans did declare war on them on the 1st as well but we don't have the need for the German player to declare war on Russia either as it is already at a state of war in game. Austria-Hungary and Serbia are of course also already at war as well, and since the 28th of July before the start date of the game.
2) On the other hand, Luxembourg is really not formally invaded until the 2nd, and you could logically group them in with the Belgian invasion as part of the Schlieffen plan.
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In my mind, at this point in the release, if there is a game play issue here that adding a declaration of war on Luxembourg solves, then I would think that is the primary case for this and players would have to accept the addition of a declaration of war on Luxembourg at the start of each game. If not, then it might be better to leave it as is instead of adding another declaration of war for possibly limited impact.
For example, if we are happy with where the UK currently sits diplomatically in game, and we assume that their current mobilization and leaning is already balanced and already taking into account Luxembourg (since it starts at war) then any additional effects to the UK could be problematic to game balance. If on the other hand something is felt to be missing regarding the UK then this could be a good mechanism to add that in via a DoW versus Luxembourg.
If in the end the only diplomatic effects would be on Belgium then there could be a case for that, but if it is negligible then perhaps not.
Either way it is something for us to think about one way or the other.