[Marriage of Frederick and Isabella, 13th century, source: Wikimedia Commons]
Isabella was the daughter of Maria of Montferrat, Queen of Jerusalem, and John of Brienne. Her mother died shortly after she was born, leaving Isabella to become queen at only a few days old. We don’t really know much of anything about her childhood. Her father ruled as regent, having no direct claim to the throne himself. She was probably raised in the city of Acre. Though she was titled Queen of Jerusalem, the city itself had last been in her people’s hands during her grandmother’s lifetime. But plans were being made by Western European leaders to try and take it back and Isabella would come to figure in those plans.
Pope Honorius III felt that the Kingdom of Jerusalem needed a strong leader who would be invested in taking back the lands that had been lost to the Saladin and his successors. The solution was to marry Isabella to Frederick, who would then be King of Jerusalem and would have a much stronger reason to actually try and take the city back. In 1225, Isabella, then aged 13, married by proxy at Acre and was officially crowned Queen. She then sailed to Italy to marry in person.
As it turned out, Isabella’s father and her husband did not get along and most of our sources were written by John’s sympathizers. They accuse Frederick of shutting Isabella up in a harem and preferring to sleep with mistresses than with his wife.* Other evidence does suggest, though, that she may have traveled around Italy with him on more than one occasion. Even married to the Queen of Jerusalem, Frederick still put off actually going on crusade. Isabella died in 1228, at the age of 16, before he had even gotten around to it, leaving Frederick no longer King of Jerusalem, but merely regent for their infant son, Conrad.
*To be fair, she was thirteen. How do we know she wasn’t glad he wasn’t sleeping with her much? Or that she didn’t ask him to leave her alone?