Izetta is powerful enough to fight whole armies, but her strength is limited.  She can only use her abilities near certain ley lines, and outside them, she's no different than a normal girl. When the Germanian army attacks in a spot far from these ley lines, the duchy is forced into a corner: how can they repel these foes without giving up the secret of Izetta's strength?
Even Archduchess Finé gave in in the end...
...حتّى الأميرة فيني استسلمت في النّهاية
Letzten Endes hat sie auch
Prinzessin Finé überredet.
Incluso lady Finé tuvo que ceder.
Incluso lady Finé tuvo que ceder.
Son Altesse a fini par céder…
Alla fine, anche la nobile Finé ha ceduto.
Até Sua Majestade desistiu no fim...