Cisalpine Gaul, and with higher numbers sent to the East, with the 18th Legion, for example, stationed in Cilicia. [Kepp., MRA , 2] Much circumstantial evidence supports this formula.
On the basis of the Keppie formula, it is highly likely that when Julius Caesar took up the post of governor of Baetica, or Further Spain, in 61 BC , the 5th, 7th and 8th legions were still based in Rome’s then two Spanish provinces, along with a 6th and a 9th. Plutarch says that there were already two legions based in Baeticathat spring, when Caesar arrived in Corduba, the provincial capital, and immediately raised a new legion in the province. [Plut., Caesar ] Following the Keppie formula, it is clear that this new unit would have been the latest incarnation of the 10th Legion. Caesar would not raise the 11th and 12th, in Cisalpine Gaul, until two years later.
Caesar himself wrote that in 58 BC he was served in Gaul by “four veteran legions”—as events were to show, these were the 7th, 8th, 9th and 10th. [Caes., GW , I , 24] Itis probable that he had asked the Senate to give him the three that had served under him in Baetica two years earlier, plus another Spanish-based legion. Caesar says that the Senate soon returned the legion complement in Spain to six. [Caes., CW , I , 85] The Keppie formula suggests that the 5th and 6th legions were left behind in Spain when the 7th, 8th, 9th and 10th joined Caesar for his Gallic campaigns.
Later events point to the Senate sending the 2nd, 3rd and 4th to the Iberian peninsula to replace the four legions given to Caesar, together with another unnamed Italian legion, possibly the Martia, while retaining the 1st Legion in Italy. We know that the 2nd Legion was definitely one of those replacement legions sent to Spain by the Senate. [ Alex. W ., I , 53] These six legions in Spain were under the overall control of Pompey the Great, who at that time governed Spain from Rome, and by 52 BC Pompey had loaned the 6th Legion to Caesar for use in Gaul. By 50 BC , Pompey had recalled the 6th Legion to Spain. [ See 6th Ferrata Legion ]
There are additional clues that support Keppie’s formula. Caesar tells us that in the summer of 49 BC , after he had accepted the surrender of Pompey’s five republican legions in Nearer Spain, he sent some of their troops to the south of France, to be discharged once they reached the River Var. He wrote that a third of the surrendered troops, “those who had homes and possessions in Spain,” were discharged at once and allowed to go home. [Caes., CW , I , 86] If the men of the 5th and 6th legions were indeed among those surrendered troops, as the Keppie formula would suggest, they had been stationed in Spain for years, and possibly recruited there.
Following the surrender of the two republican legions remaining in Baetica in 49 BC , Caesar left Quintus Cassius Longinus in charge there. The following year, Cassius “enrolled a new legion, the 5th” at Cordoba. [ Alex. W ., IV , 50] Why would Cassius choose to give the number 5 to the new legion? Earlier, at the beginning of 49 BC , before the surrender of republican forces in Spain, Pompey’s governor in Baetica, Varro, had also raised a new legion in the province, but he had never given it a number; it was known, and continued to be known, even after it defected to Caesar, as the Native or Home-Bred Legion.
At the time the Home-Bred Legion was raised, none of the numbers of the senatorial series normally allocated to legions stationed in Spain (according to the Keppie formula) was vacant—the legion numbers 5 to 10 had all been allocated to serving legions. By the following year, the republican 5th Legion no longer existed, having surrendered to Caesar in eastern Spain. Cassius was therefore free to use one of thenumbers of the surrendered legions. Because the 5th had previously been raised in Baetica, and because 5 was the first number in the Senate-approved Spanish series, Cassius was able to use that, creating
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