A group of people have been oppressed/bullied for ages.
A subset of that group of bullied people decide to violently push another group of people off their land and steal that land for themselves.
The second group of people push back with return violence.
The first group of people respond by killing even more of the second group of people and stealing more of their land.
This goes on for decades with the first bullied group gaining even more power and land, and becoming bullies themselves.