One untrue tidbit from research for Queen of the Darkest Hour: Frankish Queen Fastrada was so envious that her stepdaughter would become Byzantine empress she thwarted the girl’s marriage plans in 788.
The betrothal between Charlemagne’s eldest daughter, Hruodtrude, and Emperor Constantine was indeed thwarted, but there is no evidence that Fastrada was behind it, let alone motivated by envy. The reason for the breakup is the same as the betrothal: politics.
Let’s go back seven years to 781, when Hruodtrude was 6 and Constantine was 11, with his widowed mother, Irene, serving as regent. The couple’s parents agreed their children should wed—traditional marriage for medieval royals. The hope was to secure an alliance between the two most powerful realms in Christendom.
For the time being, Hruodtrude would stay in her parents’ household. Irene sent a tutor to Charles’s court to teach the girl Greek and familiarize her with Byzantine customs. Perhaps the agreement was that Hruodtrude would move to Constantinople when she was a marriageable age, about 12 or 13.
All was not well at the time of the betrothal. In her court, Irene harbored one of Charles’s enemies: his ex-brother-in-law Adalgis. The son of the Lombard king Charles had deposed, Adalgis had escaped to Constantinople during Frankish-Lombard war in 773-74.
When Constantine turned 16, Irene did not give up her power. And then there was Irene’s habit of calling councils His Holiness did not approve. A believer in the power of prayer to win wars, Charles would not want to risk offending God.
The situation worsened in 788, when Adalgis tried to claim the Lombard throne, impossible without Irene sheltering him. Shortly after the Franks defeated him, the betrothal between Hruodtrude and Constantine was broken.
Both Charles and Irene take credit for the breakup. Constantine was upset to lose the Frankish princess, according to both Frankish and Byzantine sources. History is silent on Hruodtrude’s sentiments.