@ATP and
@Batrix2070 -- I cannot agree with your views on the matter of Poland-Lithuania and its supposed invincibility. Yes, Poland was at the times of the Thirty Years' War much stronger than it would end up in later times, but this does not make it the bastion of super-human strength that you both fondly imagine it to be.
When it comes to the OTL strength and triumphs of Poland-Lithuania, you both ignore that in such cases, all enemies were at the same time facing multiple major enemies of their own, and were not free to dedicate their full strength to fighting Poland-Lithuania. The scenario I have posited here sees the Protestants winning the broader war, which leaves them free to then opportunistically deal with Poland in the aftermath, while all
other disputes have been settled. This is quite different from OTL, and would not go nearly as well for the Poles. That a complete dismantling of Poland-Lithuania at this stage would be extremely improbable is true... but that's of course not what I proposed. A Polish defeat against the Protestant powers (already victorious, unburdened by major conflict elsewhere) resulting in limited territorial loss? That's very plausible.
My notion of a further defeat of Poland-Lithuania in the early 18th century is a separate matter, and I've outlined in the text how other powers could be kept from interceding on behalf of the Poles. This leaves only the contention that because the Protestant powers are an "existential threat" in this ATL, Poland-Lithuania would suddenly reform and not go down the drain as in OTL. Your arguments on that point do not convince me. In OTL, Poland-Lithuania kept right on squabbling and messing up, even as the partitions were already underway. Those were an
actualised existential threat, whereas the ATL existence of a strong Protestant German Empire is "merely" a
potential existential threat. If the actualised thread does not prompt the required reforms in OTL, the potential threat may not reasonably assumed to do so in the ATL. Therefore, we must assume that Poland will -- as in OTL -- squander its opportunities for reform, and will instead decline and become vulnerable, as in OTL.
There are certainly ATLs in which comprehensive reform of Poland-Lithuania can be imagined, but as I have outlined, this particular scenario is
not set to be one of them.
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Yeah. Just as the Protestant victory sees them establishing a pretty coherent empire that leaves out the altogether too Catholic bits, I think that a Catholic victory will have the HRE much-strengthened, but with some doggedly Protestant regions in the North shorn off (becoming separate states, allied to Scandinavia). In this context, I'm particularly thinking of Brandenburg, Pomerania, Mecklenburg, Lübeck, Holstein, Hamburg and Bremen. (East Frisia and Oldenburg, conversely, would presumably be held by the HRE.)
We'd see several reversals of the trends that I have suggested in the Protestant victory scenario. Switzerland would be purged of Protestants to a considerable degree (and would stay in the HRE). Similarly, whereas in the Protestant victory scenario we see Bohemia and Hungary swing to Protestant dominance, in this ATL, they'd instead be held firmly under Catholic control. (And, more so than in OTL, they'd be purged of Protestants.)
Similarly, my Protestant victory scenario has the German Empire make gains at the expense of Poland-Lithuania, but here the reverse would be true: East Prussia would be annexed by Poland-Lithuania, and its Protestants would soon be very much encouraged to find a new home in Brandenburg. (And due to the circumstances surrounding both France and Saxony in this scenario, neither the Prince of Conti nor Augustus the Strong would easily be able to get the Polish throne, so Jakub Ludwik Sobieski -- the Austrian-backed candidate -- is likely to become King of Poland.)
In the Protestant victory scenario, many Catholics would leave the Protestant Empire. In this scenario, many Protestants would leave the HRE and head for the Protestant states up North. In particular, Saxony would be brutally punished by the Habsburgs, and the Protestants there would be smart to flee in time.
The overall success of the Catholic side would probably also affect the outcome in the Netherlands, with all of Brabant and Limburg (as well as some Catholic border regions in the country's East) being split off the Dutch Republic. (Which would become something of a Calvinist rump state.)
France would also face consequences, having joined the Protestant side and having made the wrong bet in doing so -- although France is not easily assailable. I could see them losing the wider Calais region to the Spanish Netherlands.
Spain is better off for a while, but can't easily overcome its structural issues. The Austrian Habsburgs are increasingly going to become the dominant branch, leading the powerful Catholic alliance that also keeps France encircled (much more effectively than in OTL). If the Spanish Habsburgs still end up literaly dying out, they'll make damned sure that Spain is inherited by
other Habsburgs, not by Bourbons.