The Punic Wars and also the defeat by the Germans in the Battle of Arausio showed that the Roman army lacked tactical flexibility.
This prompted changes in organization.
The old troop types, "hastati", "principes", "triarii", "velites" and "equites", and the "manipel" units that they had made up all, disappeared.
The new unit of organization was the "cohort", 600 men subdivided into 6 "centuries" of 100 men, 80 soldiers plus 20 non-combatants.
This was sufficiently strong yet flexible enough to operate as an independent tactical unit during a battle.
10 cohorts made up a legion.
The first cohort had a special status and counted 5 double-strength centuries.
Another development that needed to be responded to was the size of the Roman republic.
During the Punic and Macedonian Wars it grew beyond Italy.
The old model of a semi-professional militia army could not be maintained.
Wars were being fought up to 1,000 kilometers away from the city and some campaigns lasted the entire year, keeping the men away from their civil duties and home.
Also, the scale of the operations and the losses inflicted by enemies like Hannibal Barca
put a huge strain on the available sources of manpower.
In 107 BCE Gaius Marius took the next step and opened up the ranks to the "capite sensi", people who were too poor to pay taxes.
As they could not pay for their wartime equipment, the state started to provide this.
The money came from taxes levied on richer classes.
Soldiers had to serve for 16 years (later extended to 20), but after that could count on a pension in the form of a land grant.
People drawn from Rome's allies could gain full citizenship by serving the entire term.
So the army transformed from a militia force to a standing army.
Soldiers drilled and trained all year round and the best among them rose to the rank of centurion.
These new professional soldiers and officers provided a solid backbone,
that could save the day when the army was led by an inexperienced general.
They did not expect to return back home after a campaign, but had a sense of professional pride.
This was reinforced by giving every legion a standard in the form of an 'eagle'.
This was a rallying point for the entire legion.
Losing it was considered a major disgrace and legionnaires did everything to prevent that.
Equipment too was standardized and mass-produced.
All heavy infantry was equipped with scutum, gladius and two pila.
Each soldier had to carry his own equipment, including things like food rations for 15 days, cooking pot, a shovel, pickaxe and carrying bags.
This was so much that they were called 'Mules of Marius'.
However the men were well trained, physically fit and could cope with the load.
In battle the soldiers of course dispensed with the secondary equipment and fought with just armor and weapons.
The carrying capacity of the soldiers greatly reduced the need for wagons and draft animals and made the army march much faster than before.
Rome's new armies allowed it to conquer more land and it did.
In the century after Marius' reforms it conquered all lands around the Mediterranean and under Gaius Julius Caesar also Gaul.
Ambitious generals could win great fame and money by conquest.
This both tempted to and enabled new campaigns.
Power increasingly moved away from the senate in Rome and eventually strong men and dictators took over, plunging the state into a series of civil wars.
For a time the biggest enemy of a Roman army was another Roman army.
Eventually Augustus stabilized the situation, transforming Rome from a republic into an empire.
War Matrix - Roman professional army
Roman Ascent 200 BCE - 120 CE, Armies and troops