I have a quick question for the board here and it revolves around whether it would have been possible for Prussia do also declare war on Luxembourg during the Franco-Prussian war and have a small force head to the (Duchy?) and annex it. What I'd like to know is whether this would be a feasible action for Germany to pull off, and whether Luxembourg had any treaties that would have brought more foreign powers into the fray?
Only a couple of years earlier (1867) Prussia agreed to Luxemburgian neutrality, after France tried to buy Luxemburg. If Prussia decided to attack Luxemburg noone would trust a treaty with Prussia anymore. Other countries would fear Prussian agressian. Prussia would be a diplomatic paria and be isolated. Other, neutral, countries will form alliances to stop Prussian agression.
Besides that it could possibly cause the interventionof the other countries who signed the treaty of London (that established Luxemburgs neutrality), like the Netherlands (which to be fair wouldn't terribly impress Prussia), Britain, Austria, , etc (which would impress Prussia).
Basicly it was a risk that was not worth the reward. Good relations with neighbouring countries and being able to finish the war with France without foreign intervention is worth far more than the tiny bit of land Prussia would give Prussia. And obviously Bismarck knew that.
If not, would the introduction of Luxembourg into the German Empire have made any difference during World War I? That may seem far fetched but with it in Prussian hands would they have even needed to invade through Belgium? Or would the French have continued their fortifications to the old French border with Luxembourg?
Assuming WWI isn't butterflied away, which is likely, of course the French would extend their fortifications along the entire German border, including Luxemburg. I see no reason why they wouldn't.