John III of Sweden

John was born in 1537, son of Gustav I and his second wife Margaret Leijonhufvud, which made him half-brother to Erik XIV and full brother to the future Charles IX. He served as Duke of Finland from 1556, then led a rebellion that toppled Erik in 1569 and put himself on the Swedish throne — Erik's death followed under murky circumstances. His marriage to Catherine Jagiellon tied him to the Polish–Lithuanian dynasty; their son Sigismund would eventually rule both kingdoms. John ended the Northern Seven Years' War but dragged Sweden into a quarter-century struggle with Russia that brought minor…