This basic military form could protect Rome from squabbles with its neighbors but not against serious threats and Rome paid the price. In 483 Celts from the north defeated Rome's army and sacked Rome itself, causing the surviving Romans to realize they needed to improve their military or this might happen again. Through this experience, and conflicts with Samnites to the Southwest, Rome made an important switch in its military. It switched from the Hellenistic hoplite phalanx to a new formation based on the maniple. This formation involved using larger shields in combination with swords instead of spears. Instead of one long line that was vulnerable to the sides and rear and could easily be disrupted they formed a checkered pattern on the battlefield with squares of about 100 men. This allowed them more flexibility, as single maniples could be given seperate orders and move quickly and independently. They also allowed skirmishers to fill in the gaps between the maniples and retreat through the gaps before the enemy struck. These gaps also allowed exhausted units to move to the back of the line to be relieved by fresh ones.
The roman army was typically divided into three lines, called the Triplex Acies. The first line was made up of the Hastati who were the youngest and least experienced men with the least armor. After these men wore down the enemy they would back of so the second line of Pricepes could engage. This line was made up of experienced men in their prime who were well armed and equipped. Finally, if necessary, the third line would engage. The third line was the oldest and most experienced and also had the best armor, however their were usually the least of them. They would only enter the fight in emergencies and still used spears instead of the swords adopted by the rest of the army.
These armies also only served seasonally and Rome rarely had standing armies. They were still expected to supply their own weapons and armor