This is the 4480th most frequent Russian word.
вражеской
"Вражеской" means "hostile" or "enemy's" in English.
The word 'вражеской' is used here as an adjective modifying 'активности', indicating that the activity is characteristic of an enemy.
Надо быть осторожным в зоне вражеской активности.
One must be cautious in the zone of enemy activity.
In this sentence, 'вражеской' serves as an adjective for 'армии', thereby designating the referenced army as belonging to or associated with an enemy.
Мы анализируем стратегию вражеской армии.
We are analyzing the strategy of the enemy army.
Here, 'вражеской' modifies 'разведки', signifying that the intelligence pertains to or originates from the enemy.