Reasons for war

On the banks of a river, there were two tribes.

 The tribe upstream caught fish by across the river with their nets.

To get the fish to swim into the nets, they beat the water with long sticks, but this made the water quite murky.

The tribe down stream grew enraged. They cried out, "How can we survive when you keep making our drinking water so filthy."
Stop capturing fish, please.

The other tribe grew angry still. 

They said, "Why should we go without food only to put an end to your whining?" Both tribes eventually engaged in combat and were ultimately victorious, which caused the river to settle down and the fish to be left alone.