Greek Phalanx
The beginnings of the Roman army saw the Greek Phalanx as its main weapon and order. The Roman Army then looked a lot like the Greek soldiers which dominated the world before the Romans were the world power.
At the same time of the phalanx, the census was invented and used to create the five Roman classes, ordering from wealthiest to poor. The wealthiest class was given the most weapons, and the best among them, and the best armor. Then each class received a little less than the preceding class, until the last class was given only a sling as its weapon with no armor. The officers for the army were derived from the citizens' leaders, of course. So then it was with this order of rank that the original Roman Army consisted of 1,800 army officials, 8,000 first class soldiers, 200 engineers, 2,000 of the second, third, and fourth class soldiers, and 3,200 slingers.
This original army proved to be rather ineffective as Gaul totally obliterated the Roman Army during the 4th century. Thus to remain in control of central Italy, Rome realized it needed to make changes to its tactics of war. The first thing to be changed was the Greek Phalanx. Which proved ineffective for the Roman Army. The phalanx failed for Rome and worked for the Greeks because, the Greeks mostly fought on long plains, where as Rome is full of hilly areas, which would mean more stealthly tactics, than slow ones.