When you say "must fight," it usually means that two people or groups of people don't agree about something, and they can't find a way to settle their disagreement without physical violence (like fighting). Unfortunately, this has been happening since the beginning of time, and it still happens today. Usually, men are in charge of making the rules and deciding when fighting is needed to solve a problem. For example, if someone steals something from their neighbor, the people in charge have to decide if they should fight to get the stolen item back.