The changes made by Servius Tullius, the sixth king of Rome, survived without major changes until the 4th century BCE, and more specifically until the end of the war with the city-state of Veii, fought in the years 406 – 396 BCE. The long, exhausting but victorious war proved that the Roman army in this structure was incapable of carrying out longer war campaigns. The armament was modernized. Introduced an oblong shield (scutum) instead of a round one (clipeus). Added pilum to the spear (hasta). Iron helmets were used instead of bronze helmets. It is also worth noting that the soldiers were imposed an iron discipline and required total dedication to the republic.
General military service was introduced for men who were able to keep weapons and who had property in the age range from 17 to 46 years. In the event of a general emergency, the upper limits were raised to 50 years or more. Military service was civic and patriotic at the same time. Each of the healthy men had to take part in at least 20 hiking or 10 horse trips.
Roman army from the beginning of the 2nd century BCE reached 650,000 soldiers if we take into account the contingents of the Latin Union and other allies. However, during the war campaigns, only a fraction of the full army was used, the so-called consular army of 40,000. The rest was, you could say, an “unlimited” reserve, especially if you notice that there were no stronger troops in those days. However, it was not the numbers that gave the Romans an advantage on the battlefield. Roman troops dominated the enemy primarily with weapons and tactics. The Roman army during the monarchy was limited to the Greek phalanx and no attention was paid to newer fighting techniques. It was only in the Republic that people began to follow the principle: “One should learn from the enemy” (licet et ab hoste doceri). It was decided to reform the Roman army. First of all, attention was paid to the stiffness of the phalanx. As a result, the so-called manipulative tactic was taken from the Samnites, thanks to Marcus Camillus (turn of the 5th and 4th centuries BCE). The entire legion was divided into 30 maniples (manipulus), which formed a battle line of three lines: hastati, principes, triarii. There were gaps between the individual manipulators in each array, which were hidden by the manipulators of the next row. The entire army structure on the battlefield is called Triple Form (acies triplex). Before each heavy-armed formation, there were velites, i.e. light infantry (spearmen, archers). There was cavalry on the flanks, grouped into 10 squadrons (turmae), protecting the main army from being encircled.
The tactic of fighting the enemy was very well thought out. The first two arrays, i.e. hastati and principes, were throwing javelins, the so-called pilum into enemy units. A sword was used in close combat while protecting the body with a shield. If the first two ranks were unable to defeat the enemy, or there was a threat of a collapse of the formation, the last formation, triarii, entered the fight. This situation was described as: “It has come to the Triarii” (res ad triaros redit).
The war with Hannibal was preceded by a number of changes in the tactics of Roman troops, which until then, after all, used rather the Greek technique known as the Macedonian phalanx. Defeats at Lake Trasimeno in 217 BCE and at Cannae in 216 BCE remarkably showed the little flexibility of manipulators. They operated in a compact, manipular array, and therefore the army was very stiff. This tactic was dropped by Publius Cornelius Scipio the Elder with the nickname Africanus. He took part in the Battle of Cannae as a military tribune. He had witnessed the defeat with his own eyes, and perhaps it was this experience that led him to conclude that it was necessary to change the tactics of warfare. The Romans continued to use the Greek experience. They still believed that war was not one battle to win the war, but rather long-term action. At the same time, however, they also learned quickly and did so by introducing their own ideas for waging war. Scipio made this assumption fully demonstrated for the first time in the Battle of Baecula in 208 BCE. Then, facing Hasdrubal, Hannibal’s brother, instead of compact manipular ranks, he used the technique of operating loose troops – cohorts, each with 3 manipulations. This meant an increase in the role of the leader in battle, who could show greater initiative. As a result, the commander had to adapt the battle plan depending on the terrain and circumstances. Thanks to the new technique, the legions became more mobile and much more effective in hostilities. Earlier, the commander gave the order to start the battle, which was fought according to the usual patterns, and the commander very often lost control over the course of the battle. The crowning achievement of this new technique of warfare was the Battle of Zama in 202 BCE, where Scipio had a great winning by beating Hannibal himself. It was a real revolution in the Roman military, which in later years brought the Romans many military successes.
In addition, during the Punic Wars, the Roman fleet was developed: its numbers were increased, and the tactics of naval combat were developed. This allowed Rome to become the hegemon of the sea in the future.
The military reform of Gaius Marius in 107-102 BCE was of great importance for the Roman army, and especially for the republic.
After the end of the conquest of Italy, the Romans were able to mobilize 700,000 soldiers (Romans and allies). It was the force that decided even the bloodiest wars. Neither Carthage nor Macedonia has been able to raise an army that is nearly as large. The Romans ‘opponents have always been amazed at the Romans’ ability to raise a new army. These possibilities were decisive in the victorious wars against the Samnites and Pyrrhus. Even the defeats in the war with Hannibal at the beginning of the conflict did not exhaust the reserves of the Roman army, while Hannibal had limited opportunities to make up for shortages in the army.