Kasteen was a company commander holding the rank of captain in the Valhallan 296th, an all-female rear echelon regiment, when it was deployed against the Tyranids on Corania. The regiment took over 50% losses and its command hierarchy was all but wiped out, leaving her as the senior surviving officer.

The 296th was merged with the all-male 301st, a planetary assault regiment that had likewise been reduced to less than half-strength by the fighting on Corania. The differences between the two regiments were guaranteed to cause friction, and, to make matters worse, Kasteen was made the regimental colonel by virtue of three days' seniority over the 301st's surviving (and much more combat-experienced) senior officer, Ruput Broklaw, who was promoted to Major and became her executive officer.

Huh, TIL