It is not an easy world, and there are always some people who want to take over land or to disrupt normal living in some way. So every country in the world has to have some way of defending the general will of the people. This is what the army (along with air force, navy and so on) does today in most countries.

You can find examples of the ways that different armies fought in many books. The Ancient Egyptians were hardened warriors. Alexander the Great had an army that set out to invade, and the defending army was not strong enough to stop him. The Romans also had an invading army because they wanted to make an empire. You will find other examples throughout history.

Armies have nearly always been made up of a core of paid soldiers (regular army) along with some part timers (army reserve) except in times of real emergency, when all able-bodied people are called up (conscripts). The last time this happened in Britain was during the Second World War.