Wow, lots of good answers so far! I'm glad to have started a topic to get everyone's juices flowing
So, if I'm understanding the collected responses correctly, the Commonwealth had some things going for it, but it needed a lot of work. Short of ASB, or a very ahistorical complete destruction of Muscovite-Russia, Prussia, or Hapsburg Austria, the Commonwealth would need to;
* implement the early reforms of the
Ruch egzekucyjny
* successfully enforce the Treaty of Hadiach, averting the worst of the Khmelnytsky Uprising
** As an addendum to this, the Commonwealth would also have to be more successful in the Russo-Polish War of 1654–1667
Achieving either one or both of those points, the Commonwealth would later be more successful resisting the Swedish Deluge and subsequent incursions and divisions as per OTL. This seems to be the major turning point, and any POD past this point, and correct me if I'm wrong, would still appear to lead to a highly weakened and eventually fragmented Commonwealth.
So, how do any of these changes happen? While maintaining the Commonwealth structure - no cheating via a personal union under a Jagiellon dynasty
Also, I think its very interesting no one even touched the idea of a Polish-Lithuanian-Muscovite Commonwealth
Some specific points;
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What we hence need is a PLC (which would be best described as Aristocracy - while it used republic it wasnt really one because it had a king, and it surely wasnt a democracy. Nobles democracy is even a contradiction in terms)
I knew and was hoping someone would comment on this. I didn't want to bog down the first post talking about the Commonwealth's governmental system, but I think it is important to the question at hand. To quote Wiki;
The foundation of the Commonwealth's political system, the "Golden Liberty" (Polish: Złota Wolność, a term used from 1573), included:
* free election of the king by all nobles wishing to participate;
* Sejm, the Commonwealth parliament which the king was required to hold every two years;
* pacta conventa (Latin), "agreed-to agreements" negotiated with the king-elect, including a bill of rights, binding on the king, derived from the earlier King Henry's Articles;
* rokosz (insurrection), the right of szlachta (nobles) to form a legal rebellion against a king who violated their guaranteed freedoms;
* liberum veto (Latin), the right of an individual land envoy to oppose a decision by the majority in a Sejm session; the voicing of such a "free veto" nullified all the legislation that had been passed at that session; during the crisis of the second half of the 17th century, Polish nobles could also use the liberum veto in provincial sejmiks;
* konfederacja (from the Latin confederatio), the right to form an organization to force through a common political aim.
The Commonwealth's political system is difficult to fit into a simple category, but it can be tentatively described as a mixture of:
* confederation and federation, with regard to the broad autonomy of its regions. It is however difficult to decisively call the Commonwealth either confederation or federation, as it had some qualities of both of them;
* oligarchy, as only the szlachta—around 10% of the population—had political rights;
* democracy, since all the szlachta were equal in rights and privileges, and the Sejm could veto the king on important matters, including legislation (the adoption of new laws), foreign affairs, declaration of war, and taxation (changes of existing taxes or the levying of new ones). Also, the 10% of Commonwealth population who enjoyed those political rights (the szlachta) was a substantially larger percentage than in any other European country; note that in 1831 in France only about 1% of the population had the right to vote, and in 1867 in the United Kingdom, only about 3%;
* elective monarchy, since the monarch, elected by the szlachta, was Head of State;
* constitutional monarchy, since the monarch was bound by pacta conventa and other laws, and the szlachta could disobey any king's decrees they deemed illegal.
So, its very hard to pigeon-hole the Commonwealth into a very small and neat box in regards to its polity. It truly was unique in European and World history.
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If I could add my 3 dinars, Susano said the Deluge was the greatest blow, and solving the Cossack issue wouldn't help much. Well, Deluge was the greatest blow indeed, but the Khmelnitsky Uprising was devastating too. As was the Russian invasion it triggered. I also read that Charles chose the Commonwealth as the target of his invasion because of it's weakness, he considered Russia too.
I say, creating of a separate Ruthenian/Cossack unit in the PLC, may very well bring many positive changes. However, it will be hard, very hard to force it on the nobility, who was opposed to such changes. Perhaps an earlier uprising, when everyone else is busy could help.
Also, it wasn't the Ruthenian voivodship that would be given a duchy status, but three easternmost voivodships of the Polish Crown: Kiev, Bratslav and Chernigov. Ruthenian voivodship lied around Lwów/Lviv/Lemberg/Lwow/Ilov And I don't think it would have weakened the Crown, quite the opposite.
Other than that, slowing the reforms so that PLC lasts until Napoleon is a decent way to keep it alive, and a plausible one.
Thank you for clarifying all that, especially regarding the Ruthenian voivodship. Yes, the Uprising was really the opening, IMO, for many of the Commonwealth's neighbors needed to make inroads into Central-Eastern Europe. No uprising, or a smaller one, leading to a successful Polish–Lithuanian–Ruthenian Commonwealth and a victorious Commonwealth in the Russo-Polish War would most likely make Musovite-Russia seem weaker, and instead lead King Karl X Gustav to attack there instead of the Commonwealth. Or perhaps instead of launching an attack across the Baltic, Gustav moves against Denmark earlier than he did IOTL.
Would would you say would be the repercussions of the Commonwealth lasting, even as a minor power, into the Napoleonic Era? As I asked earlier, do you think the Commonwealth would support the French Revolutionaries (at least in the early stages)?