This is the 14941st most frequent Russian word.
подконтрольной
Subject to control; controlled.
In this context, "подконтрольной" qualifies the noun "территории" (territory) indicating that it is under someone's control or jurisdiction.
Подконтрольной территории нужно было обеспечить охрану.
The controlled territory needed to be secured.
In this sentence, "подконтрольной" describes the state of the situation in terms of being managed and within the limits of control.
Ситуация в городе осталась подконтрольной.
The situation in the city remained under control.
Here, "подконтрольной" is an adjective describing an entity or organization as being controlled or supervised by another.