Nowadays we are used to having armies, navies and an airforce to protect ourselves. That is, in part, because there are so many of us around that sometimes a group of people want to take over land that someone else already has.
In the Stone Age and the Bronze age, people probably did not think this way because there was room for all. But by the Iron Age there were enough people in Britain for tribes to claim land as their own. Britain was carved up, and tribes started to have warriors to defend them.
