can you explain the stark civil war/succession crisis that happened when beron stark died?

Ok, settle in, because this is going to get byzantine.

The background of this is that Cregan Stark was a serial monogamist with a gift for children living to adulthood. 

  • With his first wife Anna Norrey (who must have died young for reasons that will become clear in a second), Cregan had his firstborn son Rickon, who married Jeyne Manderly and had two daughters before dying outside Sunspear in the armies of Daeron the Young Dragon. 
  • Cregan married a second time to Alysanne Blackwood after the Dance of the Dragons, and had four daughters (doesn’t seem like any of them had issue). 
  • He then got married again to his cousin Lynara Stark (although where she came from is a bit unclear, but it must have been back a few generations at least since she wasn’t the descendant of his uncle and his grandfather only had two sons), and had four sons with her. 

This caused something of a dynastic issue, because Rickon’s daughters Serena and Sansa had a claim that arguably would supersede those of his living sons and at least Serena was married into the Umbers so she had political backing. Meanwhile his living sons also had significant political marriages: Jonnel (the oldest) was married into the Ryswells, and Brandon (the youngest) was was married into the Karstarks (but sleeping with a highborn crannoglady from House Fenn). 

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As the Peter O’Toole in the Lion in Winter that he was, Cregan dealt with the situation by marrying his relatives to one another. Serena Stark’s Umber husband died, so that let him marry her to Edric (his second-oldest), and Sansa was married to Jonnel, whose Ryswell wife had also conveniently passed away. Problem solved, right?

Except that Jonnel died without issue (which leaves Sansa a loose end), then Edric had twin sons and two daughters (who married into the Umbers and Cerwyns, and had issue), except that Winterfell didn’t pass to any of them (the sons may or may not have been alive), instead it passed to Barth Blacksword (the third-oldest) and from him to Brandon. I don’t imagine this went over well with Edric’s wife, who had now been passed over twice and who had the Umbers and Cerwyns on her side and the Manderlys (although they might have been temporarily peeled off when the Manderlys married Rodwell Stark), but it’s not like the Karstarks were about to step aside when they had an adult male Stark with two half-Karstark sons waiting to inherit. 

Beron was the youngest of those two sons, and was mortally wounded fighting Dagon Greyjoy’s Ironborn, leaving behind five sons who were probably around Egg’s age at the time. However, alongside Lorra Royce, Beron’s widow, you probably also had Arrana Stark and Aregelle Stark (the daughers of Edric and Serena) both of whom had issue and a claim that was undeniably better in everything but proximity, you might have had Lonny Snow or the Blackwood daughters still kicking around. 

So you have a nasty combination of competing claims and competing power blocs. 

seguemarran Asks:

How come that the boundaries of the old kingdoms and family lands in Westeros seem to be so unchangeable? Has there ever been something like a dynastic union in order to enlarge family dominions? If, for instance (not interested in this particular speculation), Sansa came to be the only heir to the North, the Riverlands and then married Robin Arryn, wouldn’t that make a huge kingdom then united by her children? Thank you!

I don’t think it’s accurate that the boundaries are unchangeable – the histories of the various kingdoms from WOIAF show the opposite, with the borders of the various kingdoms flowing hither and yon repeatedly throughout history. In addition to the gradual and often painful process of the unification of each of the Seven Kingdoms from the hundreds of petty kingdoms that once existed, we know of a good deal of back-and-forth. The Durrandon conquest of the Crownlands and the Riverlands, the Ironborn’s conquest of much of the west coast of Westeros and the Hoare King’s conquest of the Riverlands, Gyles III Gardener who conquered most of the Stormlands before falling prey to tall poppy syndrome, the North’s attempts to conquer the Sisters and the Fingers, etc. 

As to your question about dynastic union, there’s plenty of internal examples: the Stark’s dynastic marriages to the Barrow Kings, Garlan II Gardener marrying into the Hightowers and gaining Oldtown for the Reach, the dynastic union that allowed Meryn III to successfully incorporate the Arbor into the Reach, and so on. 

However, there is a Westerosi custom that says that one man shouldn’t control more than one great seat of power, both out of respect for the balance of power, and the feudal limitations of trying to administer geographically separate fiefdoms. That likely has limited the reach and extent of dynastic marriages.