system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does every country have a flag?

Every country has a flag, areas within countries have their own flags, groups of countries sometimes have a flag... How did the human tradition of representing areas and groups of people by using colors on a piece of cloth start?
gpt
It started in antiquity with military units. You had battle standards that designated specific armies and could even be used to signal for specific attacks or types of retreat. 

The tradition shifted to represent specific nobles when we entered feudal periods in society and then countries when we entered the Nation State periods of society. They also became very important when we began sailing ships as they identified the nationality of a vessel at a distance.