Fight referred to Arjuna's dharm.
When this instruction is applied to others it means that they must serve their own particular individualized dharma.
One's dharma existed in potentiality before one is born on earth. One's dharma is the essential fabric of one's subtle body (i.e. one's spirit).
A person's dharma is those very deeply seated ideas, beliefs, interests, needs desires and aspirations which one is driven to express and materialize on earth. This is the fabric of one's own spirit which existed before one was brought forth to earth in a physical human body. One was predestine by God to be born on earth because this set of spirit based ideas, beliefs, interests, needs desires and aspirations which constituted one pre-existing spirit were relevant to God's direction for earth at that time.
This is the reality of all being who are brought forth to earth from the heavenly realm. All such being have very preordained mission on earth. These mission are absolutely in line with the very fabric of each of their pre-existing spirits or subtle bodies. These mission make up each person's individual dharma.
A person's spirit changes. In fact a person's spirit is constant state of evolving. A person's spirit changes when the very deeply seated ideas, beliefs, interests, needs desires and aspirations within their spirit changes. The highest aim of life is to changes these very deeply seated ideas, beliefs, interests, needs desires and aspirations, as rapidly as possible to one that omniscience or as close to omniscience as possible.
Whiles this can happen rapidly during one current life time, it is assumed that for most people this is will not change so rapidly so as to void the very deeply seated ideas, beliefs, interests, needs desires and aspirations of one's pre-birth spirit.
Therefore although this world is all illusion, you were brought forth to earth in a human physical body because your very deeply seated ideas, beliefs, interests, needs, desires and aspirations are founded on and rooted in, illusions. And you will not be happy if you do not act according to you own dharma which is founded on your very deeply seated ideas, beliefs, interests, needs, desires and aspirations, which for all intent and purposes are illusions.
The excepting to this is if your own deeply seated ideas, beliefs, interests, needs, desires and aspirations become more enlightened then what was dentine to be your dharma could change. If this change is enlightening enough so that you deeply seated ideas, beliefs, interests, needs, desires and aspirations no longer have to do with things on earth then you may have to die . . . . for earth and its illusion would not longer be relevant to you.