Krishna tells Arjun, "There is no time, past, present or future, when you, I and these rulers on the battlefield are not present." He further adds that the 'material side' of the indestructible eternal 'living entity' is certain to perish and therefore the battle ahead must be fought. This eternal 'living entity' is known by many names such as atma , chaitanya , soul and consciousness. Krishna refers to the same as Dehi .
Krishna starts with the essence of creation and speaks of a 'living entity', which is indestructible, immeasurable, pervades all and is eternal. Secondly, the same eternal entity has a material side which invariably perishes. When Krishna mentions about rulers he is referring to that 'living entity' in them, which is indestructible and eternal.
Essentially, we are made up of two parts; body and mind which would invariably perish. They are subject to polarities of pleasure and pain; and Arjun is going through that pain. Second part is Dehi which is eternal. Krishna's emphasis is to realise this and stop identifying with body and mind( asat ) and start identifying with Dehi( sat ). Buddhatva (enlightenment) is when identification itself drops, which is an experience and can't be explained in words.
The part of the Gita where Krishna asks Arjun to fight is the most difficult part to understand. Some say that the Kurukshetra war never took place and is only a metaphorical representation of our everyday struggles. It is unlikely that the war would have come to an end by Arjun’s withdrawal from it. Krishna advocates facing battles armed with weapons of awakening and realization. Krishna knows that with Ahankaar (Aham-karta) Arjun will be a permanent slave to despondency even if he withdraws from the war. Krishna, therefore, asks him to realise Sat and fight the battle.
Source - Daily World