It's an inevitable part of historical research that one will be reading a book about one event or person, and find a secondary individual that also sounds interesting and draws your attention away. While reading Alison Weir's book on Katherine Swynford (which I recently reviewed) I came across Constance of Castile, the wife to whom Swynford was "the other woman".
Constance was the daughter of King Pedro/Peter "the Cruel" of Castile. She was technically illegitimate, her mother was the King's mistress, but after repudiating his actual wife Peter claimed to have married his mistress in the past and declared that this then made Constance his legitimate daughter and therefore his heir to the Castilian throne. Unfortunately not everyone saw it that way, and Peter's cruelty became his undoing when he was overthrown by his illegitimate half-brother Henry of Trastamara. Peter fled to Gascony with his daughters, and appealed to Edward "The Black Prince" of England for help.
Edward agreed to help but with certain conditions, which included Constance and her sister Isabella being kept in "honourable captivity" by the English, as hostages for their father's good behaviour. When Peter died the throne was officially claimed by his half-brother Henry, and the girls were left with very little. In to this problematic situation stepped John of Gaunt, who married Constance while his brother Edmund picked up Isabella. John's first wife, Blanche of Lancaster, had died a few years before leaving him with a brood of children that needed a mother.
Through Constance's claim John hoped that he could gain a throne of his own. He appears to have been utterly loyal to his father King Edward III, his brother Edward the Black Prince and then his nephew Richard who inherited as the Black Prince died before their father. He didn't want the throne of England, but he did want to be King of something. He and Constance though do not appear to have been a well-matched couple, John was an active man who took a great deal of interest in managing his lands and looking after his and Blanche's children. Constance appears to have preferred a more quiet life with the ladies she brought with her from Spain. Alison Weir points out that proof of this comes from the fact that they only had two children in their twenty three years of marriage, a daughter called Catherine and a short-lived son named John. Likewise during his first marriage John of Gaunt was faithful to Blanche, whereas while married to Constance he had at least one long-term affair (Katherine Swynford) and probably other brief ones.
In the end Constance never got her father's throne despite her and John's best efforts. When she died in 1394 she was buried in England, apart from a few months in Castile with her husband while they tried to reclaim the throne she never returned to her homeland. But her daughter Catherine took her place for her, as part of the deal to stop John and Constance claiming the throne Catherine was married to King Henry's son and became Queen Catalina of Castile. In time Catalina was also to become the great-grandmother of a famous English queen, Catherine of Aragon.
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