Fernando married Jeanne on January 1212 in Paris. He was the nephew of Jeanne's great-aunt-by-marriage Matilda of Portugal.
While on their way to Flanders the newlyweds were captured by Jeanne's first cousin Louis (the future Louis VIII of France), eldest son of Philip Augustus and Jeanne's aunt Elizabeth of Flanders. Louis' aim was to acquire his dead mother's dowry, a large piece of Flemish territory including Artois, which Jeanne's father had taken back by force after Elizabeth's death.
Released after this concession, Jeanne and Fernando soon joined the old allies of her father, king John of England and Emperor Otto IV, in an alliance against France. They were decisively defeated at Bouvines in July 1214, where Fernando was taken prisoner.
Fernando was to remain in French hands for the next 12 years, while Jeanne ruled alone.
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