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Eric of Pomerania, King of Denmark, Sweden and Norway, was engaged (at a time) to Isabelle de Bourbon, daughter of Louis II, duc de Bourbon and Anne, dauphine d'Auvergne, and niece of King Charles V of France. Unfortunately for reasons unknown to me, the match was called off and Eric was married to Philippa of England instead, with the possibility of a match between his sister and the future Henry V.

Eric had no children with Philippa, and as early as 1417 passed an act recognizing his nephew, Bogislaw of Pomerania as heir to the Kalmar thrones.
Isabelle, on the other hand, became a nun, dying in the 1450s.

Now, what if Eric had married Isabelle? Would the Kalmar Union be pro-French instead of pro-English in the 100 Years War? I can't remember exactly, but I think Margarethe I made it very plain that the Kalmar Crowns would not support England in the war (I could be wrong, I'm away from my sources at the moment).

What effects might there be if Eric marries Isabelle and has issue? Might we see a "mad" king of Denmark, Sweden & Norway further down the line like Charles VI and Henry VI?
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