enemy-n; 3 Senses

Sense Number 1: an opposing military force

Commentary: ENEMY[+entity][+group][+force][+military][+opposing]
NOTE: refers to a body of people in a military force.

The enemy attacked the city at dawn.
The scout reported seeing enemy troops along the ridge.
It was a tactic used by enemies of the Allies during World War II.

WordNet 3.0 Sense Numbers: 1

Sense Number 2: a personal adversary, rival

Commentary: ENEMY[+entity][+person][+adversary]
NOTE: refers to an individual who is a rival, opponent, adversary

Over the years he has made some political enemies in Washington.
Mary didn't want to make an enemy of Sally at work.
He has been an enemy of the school voucher program from the beginning.

WordNet 3.0 Sense Numbers: 2, 4

Sense Number 3: something harmful or injurious

Commentary: ENEMY[+relation][+role][+harmful]
NOTE: refers to anything that serves a harmful or injurious role

Alcohol was his worst enemy.
This chapter describes various enemies of the immune system.
They were being tried as enemies of the state. (injurious to the political system)
She is her own worst enemy. (she's harmful to herself)

WordNet 3.0 Sense Numbers: 3