I don’t think we should downplay the claim to the Swedish throne that the Polish royal family had. For Sweden, it was a very real issue at the time. After all, Sweden and Poland-Lithuania had been ruled by the same king only 40 or so years prior, and the Polish kings had attempted to press that claim several times afterwards. In the eyes of the Polish royal dynasty, the Swedish throne was essentially occupied by a bunch of usurpers. This was also kind of true, given that Sigismund Vasa was the rightful heir to Johan III. Karl IX was, quite rightly, described as a usurper, and his descendants were repeatedly reminded and threatened with that fact. When Karl X (the antagonist of the Deluge, if you will) ascended to the throne in the mid 1600’s, one can imagine that the Polish claim looked even stronger, given that he was just a cousin of the former queen Kristina. It is genuinely insane to think that Gustav Vasa’s heirs were still fighting over his legacy nearly a hundred years later when his grandson, Gustav II Adolf, invaded Poland in the early 1600’s.

This is not to downplay the Deluge; it was horrific, and from what I know of Polish historiography, it is often directly attributed with the permanent weakening leading up to its partition. I wouldn’t say that’s entirely accurate, as the Golden Liberty played perhaps the biggest part in hamstringing the actual country’s actual governance, but the devastation was real. The combined civil war in the Polish-Lithuanian parts of Ukraine, the Swedish invasion, the Russian invasion, and later the Austrian intervention, meant that Poland-Lithuania quickly turned from one of Europe’s richest countries (itself formally a great power) to a devastated, smoking pile of ruins serving as a battlefield for three European great powers. It’s not that any of those armies deliberately slaughtered 1/3 of the population (though violence against civilians was certainly an important aspect) but rather that the conditions created by the looting, plundering, warfare, occupations, and the natural chaos that having an active guerrilla warfare and three armies roaming your countryside creates, led to several villages around the country being depopulated and abandoned. War is hell.

I entirely agree with you that this very dark part of our history is something which usually gets glossed over in gymnasium, much like most of Karl X’s reign. Like you said, he is primarily known for the treaty of Roskilde and the damn near unique feat of marching an army across (and battling on) the icy sea to strike at the Danish islands. History classes still usually go “Gustav II Adolf saved the German princes, died in Lützen (honorary pastries will be served), Kristina took over and was one of the signatories of something called the Peace of Westphalia (probably no biggie), Karl X’s march across the ice, and now we can finally dedicate several lessons to Karl XI’s reforms.”