Ones at the gate, maybe

• Allies, to the Axis
• The spies in "Spy vs. Spy" comics, e.g
• Battling sides
• Bosses of video games, e.g
• Those opposed
• The Sith, to the Jedi
• The Blue and the Gray, e.g
• Some are bitter or sworn
• "Forgive your ___, but never forget their names": John F. Kennedy

Wiktionary Word definitions in Wiktionary
n. (enemy English)