This is the 329th most frequent Korean word.
군
The Korean word "군" can mean "county," "military," or be a suffix indicating a young man or gentleman, depending on the context.
Here, "군" is used as an ending particle to express the speaker's personal discovery or realization.
이 산은 정말 멋있군.
This mountain is really beautiful.
In this sentence, "군" refers to an army, indicating a group of military forces.
그 군은 예전에 강력한 힘을 가졌었지.
That army used to have great power in the past.
Here, "군" refers to a county, which is an administrative region in South Korea.