Negative emotions such as hate, anger frustrations, etc are used to motivate and control citizens/soldiers to kill the enemy.  The enemy is outlined in the below motivational aspects of war. 

The motivation for war, over the eons is:

A.

One group wants what the other has: whether this be money, gold, resources or land.

B.

The difference in ideologies over: religious beliefs or personal beliefs.

C.

The difference in ideology over life styles.

D.

The difference in people themselves whether it is biological, physical or emotional.

E.

The frustration of powerless people to have power over another whether this is individual or group or nation orientated.

F.

The differences in moral values  such as:  birth control methods,  abortions, sexual freedom, creates strong reactions.

G.

A contest of ruler’s ego’s who control entire populations by inventing an anger to satisfy their wounded egos.

H.

An individual or nation that feels that its rights, finances, or any other situation has been compromised by another. 

I.

Fear of attack brings preemptive strikes. 

J.

Attack brings retaliation. . 

In other words anything that creates an unsolvable resolution amongst others

is the flint to light the fire of war.

As long as there is a difference that cannot or is not resolved by effective communication;  anger results from the frustration.  Anger motivates the fighting amongst people. 

Anger is the resulting explosion of negative emotions .  Anger is the fuel that starts the fire of War.