A fight between opposing forces in a war

A fight between opposing forces in war is called a battle.

Explanation:

A war is a larger concept which usually comprises of multiple battles fought between the forces of two distinct geographic regions. These battles intend to cause bit-by-bit damage to the opponent in order to make him weaker and weaker. A war is fought over a long period of time and involves negotiations off-field also. A battle, on the other hand, does not involve negotiations as it is only characterized by fighting physically to win over the other.