There is not one answer that fits all situations.
Tribalism can bring people together to work for common goals and with everyone's interest in mind. Not all tribal communities are warring.
Many warring Nations are not run tribally.
What causes war?
The war begins inside ourselves, when we begin to separate and judge Ourselves and others as different than each other.
Yet, the true cause of war in the world is struggle for power and money. War is a product of evil and it is designed to separate us and conquer all with fear.
Those in power who create wars for their selfish gains will also create illusions to distract from the truth.
Tribalism could be used to unite and/or to separate.
It depends on the intentions and hearts of the people involved.