When pavement ants (the little reddish brown ants you see in sidewalk cracks) have intra-species war they use the concept of a one-to-one correspondence to determine who has a larger army.

Ants pair off locking jaws with another ant of similar size.

Any leftover ants from the larger colony will gang up two on one against the other colony.

Then based on things only ants know either they all go home OR one colony overwhelms the other.

But most of the time only a few ants die.