Zooming out even further, why Roman victory in the Roman-Seleucid War? I think there are a few clear factors here.
Ironically for a post covering land battles, the most important factor may be naval: Rome’s superior naval resources (and better naval allies), which gave the Romans an enormous operational advantage against Antiochus. In the initial phase, the Romans could get more troops to Greece than the king could, while further on, Roman naval supremacy allowed Roman armies to operate in Anatolia in force (while Antiochus, even had he won in Greece, had no hope of operating in Italy). Neutralizing Antiochus’ navy both opened up options for the Romans and closed down options for Antiochus, setting the conditions for Roman victory. It would have also neutered any Roman defeat. If Antiochus wins at Magnesia, he cannot then immediately go on the offensive, after all: he has merely bought perhaps a year or two of time to rebuild his navy and try to contest the Aegean again. Given the astounding naval mobilizations Rome had shown itself capable of in the third century, one cannot imagine Antiochus was likely to win that contest.
Meanwhile, the Romans had better allies, in part as a consequence of the Romans being better at getting allies. The Romans benefit substantially from allied Achaean, Pergamese and Rhodian ships and troops, as well as support from the now-humbled Philip V of Macedon and even supplies and auxiliaries from Numidia and Carthage. Alliance-management is a fairly consistent Roman strength and it shows here. It certainly seems to help that Roman protestations that they had little interest in a permanent presence in Greece seem to have been somewhat true; Rome won’t set up a permanent provincia in Macedonia until 146 (though the Romans do expect their influence to predominate before then). By contrast, Antiochus III, clearly bent on rebuilding Alexander’s empire, was a more obvious threat to the long-term independence and autonomy of Greek states like the Pergamum or Rhodes.
Finally, there is the remarkable Seleucid glass jaw. The Romans, after all, sustained a defeat very much like Magnesia against Hannibal in 216 (the Battle of Cannae) and kept fighting. By contrast, Antiochus is forced into a humiliating peace after Magnesia, in which he cedes all of Anatolia, gives up any kind of navy and is forced to pay a crippling financial indemnity which will fatally undermine the reign of his successor and son Seleucus IV (leading to his assassination in 175, leading to yet further Seleucid weakness). Part of this glass jaw may have been political: after Magnesia, Antiochus’ own aristocrats seem pretty well done with their king’s adventurism against Rome.
But at the same time, some of it was clearly military. Antiochus didn’t have a second army to fall back on and Magnesia represented essentially a peak “all-call” Seleucid mobilization. A similar defeat at Raphia had forced a similarly unfavorable peace earlier in his reign, after all. Part of the problem, I would argue, is that the Seleucids needed their army for more than just war: they needed it to enforce taxation and tribute on their own recalcitrant subjects. As a result, no Seleucid king could afford to “go for broke” the way the Roman Republic could, nor could the Seleucids ever fully mobilize the massive population of their realm. The very nature of the Hellenistic kingdom’s ethnic hierarchy made fully tapping the potential resources of the kingdom impossible.
As a result, while Antiochus III was not an incompetent general, he ruled a deceptively weak giant. Massive revenues were offset by equally massive security obligations and the Seleucids seem to have been perenially cash strapped (with a nasty habit of looting temples to make up for it). The very nature of the Seleucid Empire – like the Ptolemaic one – as an ethnic empire where Macedonians ruled and non-Macedonians were ruled kept Antiochus from being able to fully mobilize his subjects. It may also explain why so many of those light infantry auxiliaries seem to have run off without much of a fight. Eumenes and his Pergamese troops fought for their independence, the Romans for the greater glory of Rome and the socii for their own status and loot within the Roman system, but what could Antiochus offer a subjected Carian or Cilician except a paycheck and a future of continued subjugation? That’s not much to die for.
Bret Devereaux, “Collections: Phalanx’s Twilight, Legion’s Triumph, Part IVb: Antiochus III”, A Collection of Unmitigated Pedantry, 2024-04-05.
February 17, 2025
QotD: Decisive factors in the Roman victory over the Seleucids
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