Why Do Ants War?

Q: During my childhood I have witnessed many ant wars. Mostly between big and small ants... is it true they are fighting for gain or is it just survival?
Vijay, Hyd, India

A: Ants fight for gain, certainly, but gain often promotes the colony's survival. In general, ants battle to monopolize food resources, to protect their nest, to gain or protect territory, or to stop other insects from stealing their food. Actually, most ant species avoid fights if they can. Army ants, however, eat the brood of other ant species, and war as a way of life.

Examples:
(the superb image links are courtesy of Alex Wild)

It's spring in Reno, Nevada, and a wild melee on the pavement. Black pavement ants spill out from two neighboring nests, and battle to expand their territories. Ants grapple in fierce combat all over the sidewalk. Closing in on the scene, we see two combatants <— heads together, antennas waving, mandibles snapping like wolves. Image.

Small black-red cone ants attack huge, red harvester ants with devious design <— to trap the giants in their nest. One harvester straddles <— red legs far apart — her tiny opponent, and chews on the cone ant's head. But a sister cone ant, her mandibles menacing, advances on the biting giant. Eventually, the cone ants win the war, trapping the large harvester ants, and confining them to their nest. The cone ants then take over the foraging area. Image.

All is peaceful at the Black Cockatoo Bush Camp in South Australia, when a column of small, black acrobat ants pass too close to a large tree-ant's nest. The heavily-armored tree ant lashes out, easily killing one small acrobat with a single bite of her powerful mandibles. But many little sisters rush the big one, squirting thick acrid foam from their abdomens, and coat their enemy in poisonous fluid. This is chemical warfare at a vicious level. Image.

Three red, army ants attack a black, soldier, pavement ant defending her colony's brood, while hordes of other army ants grab pale, whitish larvae <— the pavement-ant young. Later, feeding at her leisure, an army ant will bite the skin of a captured larva with her mandible so she can suck out the larva's blood-like hemolymph. Image.

Thus, among other things, ants fight to gain territory or foraging ground, to defend a nest or its brood, and to plunder brood.

Further Reading:

Ant Behavior by Alex Wild, Myrmecos.net
Ant fighting by Gakken, Gakken's photo encyclopedia
How do ants communicate, Myrms ant nest