The changes will already start 1864. Before IOTL the Great Powers came and sat themselves like 200-pound gorillas on Schelswig-Holstein, already Hannover and Saxony had assembled an army and sent it to Holstein. Under military pressure from the two Great Powers the middle powers then had to retreat.
Now what would a Hannover still in personal union with GB do?
So, I think we need a kind of Flow chart
First, the German-Danish War. There are three possibilities: Hannover supports Denmark (very unlikely), Hannover stays neutral (very likely), Hannover supports Prussia and Austria and insists on taking part in the war (somewhat likely). Then the German War, where again Hannover has three possibilities: Support Prussia, remain neutral, support the German Confederation (Austria), . Hence, Ill codify that like 2-2 (Hannover remains neutral in oth wars)
1: Supports Denmark. I doubt theyd support Denmark - Denmark blatantly had broken the Treaty of London, after all. Of course, it would be funny if UK and Hannover still did, because it would be mostly a land war. Prussia and Austria could easily just occupy Hannover, which then would become another possibel casus belli to use by Bismarck to start his much-desired war with Austria. Once the dust is settled 1866, territorially there will be no changes to OTL as both Hannover and Schleswig-Holstein are directly Prussian.
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2: Neutral Hannover in 1864: Well, thats very possible. Hannover itself is just a middle power, after all, and the UK wont really intervene. So its very possible King Victor is careful. Nothing changes.
2-1: 1864 neutral Hannover supports Prussia: Possible if King Victor also wants the end of the GC. Prussia wins the war, of course, just as IOTL. Of course, unlike the other North German states, Hannover cant be pressured into the North German Confederation. Oldenburg (which Hannover surrounds) was a Prussian ally, but somehow I think theyll use the chance of their geographic position to jump off Prussian domination, too. So, the NGC lacks Northwest Germany. Hannover might even push for the other German states not to be annexed, bt Prusis apulled that off so quickly I doubt Hannover can do much.
2-2: Hannover stays neutral. Prussia wins the war as IOTL, and again the NGC doesnt have NW Germany.
2-3Hannover supports Austria: Possible if King Victor wants to retain the GC (which is nice as a guarantee of sorts for Hannover, I guess). Now if Hannover only receives a minimal amount of support from GB (doesnt even need to be diretc military aid, IOTL Hannover mostly needed military supplies...) this could be interesting. Now, again seperating that in a secnario where Hannover is more successful and one where it isnt:
2-3-1:Hannover is more successful: IOTL Hannover won the first battle against Prusisa, but was then faced with way superior numbers. Now, if Hannover could hold out for a bit and rally the other German states of the region that IOTL were just steamrolled (mainly the Hesses and Nassau)... then we could see a situation where Prussia wins against Austria on the primary theatre, but loses on the secondary theatre. This would be catastrophal for Bismarck who wanted a quick end to the war. Without the middle German states being occupied and pinned for annexation, its likely he wont be able to convince King William I to have the basically white peace with Austria. And with the war going on, Bismarcks nightmare will come true and Napoleon III intervenes...
2-3-2: Its still not enough, Hannover loses: Weeelll, thats the big question, isnt it? Personally, I dont see the UK intervening, either. Parliament will be rather against it. Still, its something Prussia will have to fear. Hell, Prussia didnt annex Hesse-Darmstadt because a member of the House was at that point Tsarina of Russia. If theres even a direct personal union - yeah, Hannover will survive, with some territory annexed to Prussia (and maybe some to Oldenburg and Brunswick as well). The question then is wether Hannover will be part of the NGC. Its rather inconceivable that the British Monarch (even if it is NOT in the role of British Monarch but King of Hannover) will be under Prussian domination, but OTOH Prussia already was lenient with letting Hannover live. IMO, such ceremonial problems can be solved... per the epace treaty King Victro is forced to name a regent for Hannover who concludes the NGC business or somesuch. The end result is that the personal union is technically retained, but de facto worthless as Hannover now is in Greater Prussia, i.e. the NGC.
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3: Hannover supports Austria and Prussia: Funnily enough, King Victor might decide he needs to lessen his reputation as a foreign ruler for a stable reign to suceed, and hence be on Austro-Prussian side (and insist his troops arent sent home, either, and with an implicit UK backing of this insistion Austria and Prussia cant very well decline). Hannover then could (re)gain Lauenburg, as they already once had it, after all, while Prussia still has Schleswig and Austria Holstein.
Now Bismarck didnt care at all about who gains Schleswig-Holstein. He simply wanted a war with Austria in order to destroy the German Confederation. So he could very well let Hannover keep Lauenburg. Of course, Hannover could still follow the call of Confederal War against Prussia. Or support prussia. Really, come to think of it, the choices would still be as in 2-x
