Highly disciplined, well-organized, and fearsome, the Roman army was arguably the greatest military force in world history. Nothing embodied the might and efficiency of the ancient military more than the Roman legion—its essential building block. While its origins can be traced to Rome’s very beginnings, the legion’s heyday came in the waning days of the Republic. The transition from a citizen-soldier army to a permanent professional force changed the role and nature of this iconic military unit. Legionaries were now posted on the Empire’s frontier.
While legions (or detachments) could be moved elsewhere, frontier life caused the emergence of a specific soldier’s identity. Additionally, while all legions had one main purpose—to expand and defend the Roman Empire—each of them possessed a distinct collective identity. Every unit had its own cognomen (name), emblem, and founder. A legion’s service history was filled with achievements and trophies that fostered a sense of pride, similar to that of modern-day regiments. The legions used standardized equipment and employed similar tactics, but their performance varied. Some were consistently reliable, lasting for centuries, while others succumbed to failure.
While the Empire had around 30 legions at any given time, those numbers were duplicated over the years, since new legions emerged, while old ones were disassembled and perished in conflicts. A few legions, however, gained fame that persisted up to the present day. This is their story.
1. Legio V Macedonica: The Enduring Roman Legion
The Fifth Macedonian legion was one of the rare Roman legions that could trace its origin to the end of the Republic, while its end arrived at the dawn of the Middle Ages. This enduring legion was founded by Octavian (the later Emperor Augustus) in 43 BCE. Following the Battle of Actium, where the Fifth probably saw action, the unit became one of the 28 permanent legions of the Roman Empire. For a brief time, the unit was stationed in Macedonia (gaining its cognomen) before moving to the Danubian limes. The legion’s emblem was an eagle, Jupiter’s favorite bird.
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Like most Roman legions, the Fifth Macedonica or its detachments were transferred to crisis zones in different parts of the Roman Empire. During the first and second century CE, the Fifth saw action in the First Jewish-Roman War in Judea, Emperor Trajan’s Dacian Wars, Lucius Verus’ campaign against the Parthian Empire, and Marcus Aurelius’ war with the Quadi and Marcomanni on the Danube. The legion gained numerous distinctions for its service, most notably Pia Fidelis (“faithful and loyal”) and Pia Constans (“faithful and reliable”). The unit remained in the East, fighting against the Sassanid Persians, before being transferred to Egypt in the fourth century. Egypt became the base of the Fifth Macedonia, and it was from here that the legion went to Yarmuk in 636 to meet its doom in the fight against Arab invaders. Its endurance makes the Legio V Macedonica the longest-lived Roman legion in history.
2. Legio III Gallica: The Valiant One
The Third Gallica was probably one of the most renowned Roman legions. The unit was established in 49 BCE by none other than Julius Caesar himself. The legion’s symbol—a bull—was the animal attributed to Venus, whom Caesar claimed as his ancestor (the bull, however, was also an emblem of most legions established by the famous general).
The cognomen of the unit—Gallica—suggests that it originated from Gaul, where Caesar achieved some of his most famous victories. The main role of the Third Gallica was to serve as the backbone of Caesar’s military force in his war against Pompey the Great. It performed valiantly in the most crucial battles of the conflict—Pharsalus and Munda.
After its founder’s death, the Third was handed to Mark Antony to assist in his ill-fated war against Parthia. The legion continued its service in the Roman Imperial Period. It fought in North Africa, in the Arabian Peninsula, and for a brief time, it was stationed on the Danube. Yet, for most of its existence, the Third Gallica remained in the East. Its base was Raphanaea in the Roman province of Syria. Thus, it was one of the principal Roman army units employed in the wars against the Parthian and later Sassanid Empire. Beside the campaigns against Persia in the East and various barbarian tribes in the West, Legio III Gallica was involved in several civil wars. It is one of the few Roman legions that survived until the Late Empire and it is last mentioned in the mid-fourth century.
3. Legio XII Fulminata: The Might of the East
The Lighting Twelfth was another Roman legion established by Julius Caesar. Named after the unit’s symbol, the thunderbolt, the Twelfth could trace its origins to 58 BCE. The legion served in some of Caesar’s most famous battles in Gaul, including the Siege of Alesia. It also saw a fair share of fighting in the civil war against Pompey, participating in the decisive showdown at Pharsalus. Following victory in the war, Caesar changed the legion’s name to Victrix (“winner”). Another cognomen was later added by Mark Antony—Antiqua—meaning the “old one” (as in “reliable one”).
Mark Antony brought the Lighting Twelfth to the East to join his war against Parthia. While the campaign ended in defeat, the legion remained in the East during the imperial period, continuing the fight against the Parthian and the Sassanid Empires. The unit still guarded the Euphrates crossing next to its base—Melitene—at the beginning of the fifth century. This longevity makes Legio XII Fulminata another one of the longest-standing Roman legions on record.
4. Legio IX Hispana: The “Lost Legion” of the Roman Army
The Ninth Spanish also counts among the oldest units in the imperial Roman army. Its first mention comes from 58 BCE when the legion participated in Caesar’s Gallic campaign, but it was probably founded earlier. While in Gaul, the Ninth fought in several battles, most notably against the Nervii. Caesar was particularly impressed by the legionaries’ bravery in action. After the civil war broke out, the Ninth saw combat against Pompey’s forces at Ilerda, and in the decisive battle of Pharsalus. The legion probably got its cognomen during its stay in Spain, where it participated in the large-scale campaign against the Cantabrians.
In 43 CE, the unit was transferred to the north to participate in the conquest of Britain. It was there that the Ninth Spanish would gain its fame (or infamy). Around the 120s, the legion abruptly disappeared from all Roman army records. Possibly the unit perished in the battles on the northern frontier. Moreover, its loss might have compelled Emperor Hadrian to build the famous wall. However, more recent research has shown that in 121, the Ninth (or its detachment) was stationed at Nijmegen. After this point in time, the legion disappears from all sources. The list of legions from the reign of Marcus Aurelius (161-180) does not mention the Legio IX Hispana. This fact points to the unit’s destruction before or during his reign.
5. Legio X Equestris: Caesar’s Favorite
Formed by Julius Caesar around 61 (or 59) BCE, during his stint as the governor of Hispania, the Tenth Mounted was Caesar’s first command. Like Caesar’s other legions, the Tenth had a bull as its emblem. However, the Tenth Mounted was Caesar’s favorite and his most trustworthy legion. The Tenth accompanied the famous general when he invaded Gaul in 58 BCE. It also played a key role in the civil war against Pompey, being present at Ilerda and Pharsalus. The legion’s most famous moment was at the Battle of Munda, the final battle of the conflict. After Caesar personally entered the fray, fighting alongside his favorite soldiers and encouraging them, the Tenth pushed back the Pompeians, winning the day (and the war).
Following Caesar’s assassination, the Tenth Mounted joined Mark Antony in his Parthian campaign. The legion also took his side during the war with Octavian. After Actium, the Tenth legion was among the troops that surrendered to the future emperor. However, following their revolt, the soldiers of the Tenth were punished, with the legion’s name stripped. Legio X Equestris was subsequently merged with Legio X Gemina (“the twin”), continuing its service in the imperial Roman army as a new unit. For the rest of its existence, the Tenth was stationed on the Danube and Rhine rivers, guarding the frontier against Germanic invaders. The last mention of the legion comes from the early fifth century when it was located in Vindobona (modern-day Vienna).
6. The Unlucky Roman Legions: Legio XVII, XVIII, and XIX
Most famous Roman legions have entered into history’s annals for their glorious past, long and distinguished service, and other similar achievements. However, in the eyes of the Romans, some of the most notorious legions were the infamous ones. So, naturally, we should mention the “unlucky three”—the three legions lost in one of the most humiliating Roman defeats—the Battle of Teutoburg Forest in 9 CE.
All of the “unlucky three” were probably founded by Octavian in 41 BCE to end Sextus Pompeius’s occupation of Sicily. They later participated in Octavian’s war with Mark Antony, which culminated in the victory at Actium. The units names, sadly, are lost to history. What we do know is their tragic fate.
The Seventeenth, Eighteenth, and Nineteenth were dispatched to the Rhine in 6 CE to secure the newly conquered province of Germania. Three years later, while trying to suppress the revolt of the Cherusci, the legions were ambushed and annihilated in the battle of Teutoburg forest. Their commander and governor Publius Quintilius Varus, also perished in the struggle. While the three eagle standards were recovered during the reign of Tiberius (by Germanicus) and Caligula, the legionary emblems are not known. Furthermore, all three legions were stricken from the Roman army register, and after that, their numbers were never used.