An armed adversary (especially a member of an opposing military force).

synonyms: enemy, foe, opposition.

Roget 891: enemy; antagonist; foe, foeman^†; open enemy, bitter enemy, opponent etc. 710; back friend.    public enemy, enemy to society.   

Dutch: tegenstander, vijand, bestrijder, opponent
Polish: wróg, przeciwnik, nieprzyjaciel