Military technology is likely to be transferred to the enemy whenever it is used against them. Through battle the enemy at least learn of the existence and capabilities of the weapons and techniques used against them, and may attempt even on that basis to reproduce them. Thus Cato was said (by Pliny, NH pref. 30) to have been educated by Hannibal, as well as by Scipio. Or they may capture a specimen and/or people who know how to use it, and copy that with advice from the captive(s). If they do secure a specimen, they will also know more of its shortcomings (every weapon has some). The Nervii learned how to make siege-works by watching the Romans and being instructed by prisoners of war; Caesar elsewhere commented that they were very good at copying, and were inventive too, and some technologies could be transferred simply by intelligent copying. That, no doubt, was one method by which catapult technology was diffused, and would explain why some worked well and some did not.
The Romans were extremely adept at adopting technologies from peoples they conquered. In this process pragmatism was apparently unhindered by prejudice, and the best of ancient technology, wherever it originated, was absorbed into Roman traditions, where it met, modified, and was modified by other technologies, old and new. Their armor and weapons were in constant evolution because of contact and conflict with other peoples. For example, most forms of “Roman” helmet seem to have been based on Celtic designs, the pilum may have originated with the Etruscans, “Moorish” javelins came from Africa, and cataphract cavalry-archers were adopted from the Persian/Parthian tradition. The emperor Antoninos was nicknamed Caracalla after the Celtic or Germanic word for a type of cloak that he adopted and adapted—a full-length hoody. Sometimes the debt was explicit and acknowledged, for example, Polybios tells how the Romans first learned to build a fleet by copying a Karthaginian vessel that ran aground, and later copied Rhodian ships. Centuries later Vegetius added that experience in battle showed the Romans that their Liburnian allies’ warships were of a better design than anyone else’s, including their own, and as a result the Romans copied both the design and the name.
The Egyptians were reputed by Caesar so good at copying Roman tools and techniques that “no sooner had they seen what was being done by us than they would reproduce it with such cunning” that they seemed to be the originators. Away from the southern Mediterranean, in northern Europe, the Batavians, who were evidently much less skilled than the Egyptians at copying by sight, used deserters and captives to teach them how to make and use Roman siege engines and sheds. The Romans’ expertise with catapults meant that such “crude” machines were soon destroyed by Roman shot or firebrand, but the Batavians in due course became the Germans’ artillery experts, and other German tribal leaders asked them to build machines and siege works for later campaigns. It is likely that engineers, usually stationed by their machines, were captured if not killed whenever machines were captured; thus we are told that one of Pompey’s chief engineers, one L. Vibullius Rufus, twice fell into Caesar’s hands. Another method of technology transfer through battle was by accident, so to speak. Hanno moved Utica’s artillery to his camp, after (as he thought) chasing away the mercenaries who were besieging the town, but while he and most of his forces were celebrating in said town, the mercenaries came back and seized his camp, and thus obtained both his and the town’s artillery. Some Spaniards, after causing a Roman force to abandon its camp under cover of dark, entered the deserted camp and armed themselves with the equipment that had been left behind “in the confusion.” The Iapydes of the transalpine region used against Augustus Roman machines that they found lying around some years earlier, after a civil war clash in their territory between Brutus, on the one hand, and Antony and Octavian on the other. The tale of Bousas, who taught the Avars how to build helepoleis, siege towers, is another example.
Another vector is the arms dealer, moving either between allies, or between suppliers and customers whoever and wherever they may be. A negotiator gladiarius, procurer of swords, who pops up in the records of Mainz, was perhaps such an arms trader. This trade is notoriously secretive, as well as dangerous, and it would be naïve in the extreme to think that the paucity of evidence for arms traders in antiquity was an accurate reflection of the state of the business at the time. The largest businesses known from classical Athens were arms manufacturers (shield workshop and blade maker, respectively), and it is inconceivable that the arms trade was not flourishing in a world where warfare was more common and regular than tax collection. The Codex Theodosius laid down capital punishment for anyone caught teaching the barbarians how to build ships, but the Vandals (who had moved down from inland Eurasia) nevertheless had found out how to build a decent navy by A.D. 419. Cassiodoros meanwhile lamented that Italy (being governed by the Goths when he was writing) lacked a navy despite the abundance of timber.
To the victor went the spoils, and on surrender of a town, their catapults, along with their arms, ships, and money, were usually handed over. This could be a very effective method of acquisition of new technologies. Consider a comparative case from Hawaii. In 1790, the lightly armed American merchant ship Eleanor fired a broadside and killed about 100 natives. The natives responded by seizing the next American ship to reach the islands. They unloaded its weapons, and captured a white man (haole) who was able to teach them how to use the guns. Over the next few years they captured more ships and seized their weapons and gunpowder stores. By 1804, one of the chiefs could deploy 600 muskets, fourteen cannon, forty small swivel guns, and six mortars. Returning to antiquity, paperwork might be an asset to be exploited by the conquerors, or not, depending on the particular conqueror’s appreciation of the contents. Thus it appears that the Karthaginian libraries were given away by the Romans in 146 B.C., when they destroyed the city, making an exception only of Mago’s twenty-eight volumes on agriculture, of which the senate ordered a Latin translation be made. Polybios records that Philip V, not relying on another bout of such absent-mindedness by the Romans, kept his head in such an emergency and ordered the burning of his papers before the Romans could get their hands on them. Papers might include blueprints or other scientific or technological information of use to the enemy, as well as diplomatic documents. Certainly, Pompey recognized the value of Mithridates’ toxicology results, which arose from a program of research into poisons so successful that Mithridates was reputedly immune to all known venoms and toxins so that when he wanted to commit suicide he had to fall on his sword. Pompey ordered a Latin translation be made of them, apparently for his own use rather than that of the Roman reading public, since there is no hint that it was ever published either in its original Persian or in Latin.
Allies will copy good technology too, of course. Polybios’ belief in Greek superiority over the Romans leaks out through his text here and there. At one point he compares in detail the Greek and Roman methods of cutting and setting stakes around a palisade, and having concluded that the Roman way was better, said that if any military contrivance was worth copying from the Romans, then this was it. One could be forgiven for thinking (erroneously) that the phalanx had beaten the legion.
It is not just technology that is reproduced by enemies; fighting techniques are, too. Caesar observed that troops adopt the fighting techniques of the enemy if they fight them continuously over a long period. We are reminded of the Spartan Antalkidas’s criticism of his king, when he said that by persistently fighting the Thebans, Agesilaos had thereby provided them with the means necessary to defeat the Spartans. Agesilaos had apparently ignored a decree of the legendary Spartan lawgiver Lykourgos that forbade campaigning frequently against the same people, for that very reason.
Rome’s enemies did not always want captured ordnance, of course, and might destroy it instead. Thus the Parthians destroyed the siege engines apparently left behind by Antony in his haste through their country, including an eighty-foot-long battering ram and other equipment whose scale is indicated by the fact that it was being transported on three hundred wagons and protected by more than 10,000 troops. In the third to sixth centuries A.D., the Goths, Vandals, Huns, and sometimes even the Romans themselves destroyed more than they copied, and the western empire descended into the Dark Ages.