This is the rarest of what-ifs: a personal union that (I think) hasn't been suggested before.
But it's actually rather simple - let's say that King Frederick III of Denmark dies early in 1645 (horse trips, or something equally inconsequential), three years before he becomes king, and before he's had any kids.
Barring massive butterflies, it's entirely reasonable for his older brother Christian to also die in the next few years without any kids (as OTL), soon followed by his father king Christian IV.
IOTL his successor was Frederick III (and subsequently his descendants). ITTL it's - who? Christian IV has outlived all of his legitimate children, and while he may have several children from his morganatic marriage with Kirsten Munk it seems like (correct me if I'm wrong) the Danish Estates and Rigsrådet are going to be very reluctant to pass the throne to any of them.
Christian IV's agnatic heir is therefore actually rather far afield. He has no surviving brothers (and none of those brothers had any children) - so it passes to his eldest sister, Elizabeth of Brunswick, who is herself dead, as are all her sons (and none of them had any children, either), so it passes to her oldest daughter, Sophia of Nassau-Dietz, who is also dead, but who does have one living son (fucking finally) - William Frederick of Nassau-Dietz, who now has the closest thing to "a good claim to the throne of Denmark".
William Frederick is 34 and reasonably canny but (this is where things get a bit weird) - actively involved in Dutch politics, and even the stadtholder of Friesland, Groningen, and Drenthe at this point. He's also high up in the House of Nassau - so much so, in fact, that after the death of his distant relative William III, his great-grandson eventually ruled all of the Netherlands. And this twist in the Dutch succession can't just be "butterflied" away - if that's the case then either there's an Anglo-Dutch Union or there was no William III/Mary II or Glorious Revolution at all - there is literally no possible way for Europe to get out of this without massive changes. (This is all assuming that William Frederick is chosen to be King of Denmark, which isn't a given; but as far as I can tell, he would leap at the chance - and I don't think there's a much less "foreign" option after him?)
So if we get a Danish-Frisian Union / later a Danish-Dutch Union, how does that reshape Danish/Dutch/European politics?
But it's actually rather simple - let's say that King Frederick III of Denmark dies early in 1645 (horse trips, or something equally inconsequential), three years before he becomes king, and before he's had any kids.
Barring massive butterflies, it's entirely reasonable for his older brother Christian to also die in the next few years without any kids (as OTL), soon followed by his father king Christian IV.
IOTL his successor was Frederick III (and subsequently his descendants). ITTL it's - who? Christian IV has outlived all of his legitimate children, and while he may have several children from his morganatic marriage with Kirsten Munk it seems like (correct me if I'm wrong) the Danish Estates and Rigsrådet are going to be very reluctant to pass the throne to any of them.
Christian IV's agnatic heir is therefore actually rather far afield. He has no surviving brothers (and none of those brothers had any children) - so it passes to his eldest sister, Elizabeth of Brunswick, who is herself dead, as are all her sons (and none of them had any children, either), so it passes to her oldest daughter, Sophia of Nassau-Dietz, who is also dead, but who does have one living son (fucking finally) - William Frederick of Nassau-Dietz, who now has the closest thing to "a good claim to the throne of Denmark".
William Frederick is 34 and reasonably canny but (this is where things get a bit weird) - actively involved in Dutch politics, and even the stadtholder of Friesland, Groningen, and Drenthe at this point. He's also high up in the House of Nassau - so much so, in fact, that after the death of his distant relative William III, his great-grandson eventually ruled all of the Netherlands. And this twist in the Dutch succession can't just be "butterflied" away - if that's the case then either there's an Anglo-Dutch Union or there was no William III/Mary II or Glorious Revolution at all - there is literally no possible way for Europe to get out of this without massive changes. (This is all assuming that William Frederick is chosen to be King of Denmark, which isn't a given; but as far as I can tell, he would leap at the chance - and I don't think there's a much less "foreign" option after him?)
So if we get a Danish-Frisian Union / later a Danish-Dutch Union, how does that reshape Danish/Dutch/European politics?