The Roman republic and the Roman Empire grouper their soldiers into organized military units

It's true. They're called Legions.

Explanation:

At the peak of its power, the Roman empire was considered to have the most well-organized Military in the world. They never considered to be physically strong (like the Spartans for example), but the strategy and technology that they implement made theme extremely dominant in war.

For easier coordination, the Roman grouped their soldiers into smaller units called the Legion. Most of legion members were taken from the common citizens (such as farmers or wood workers). They usually serve as frontliners in every War.