This is the 9882nd most frequent Russian word.
агрессором
Aggressor.
The word 'агрессором' is used to describe someone taking aggressive actions in a specific conflict.
Он выступил агрессором в этом конфликте.
He acted as the aggressor in this conflict.
Here, 'агрессор' is used as a subject of accusation in international or interpersonal relations.
Некоторые страны обвиняют соседа в том, что он является агрессором.
Some countries accuse the neighbor of being the aggressor.
In this example, 'агрессором' refers to a nation or entity portrayed as having perpetrated acts of aggression in history.