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I'm wondering about the largest number of legions to take part in a single battle (no fair counting both sides in a civil war battle). What I wonder is, was there an upper limit on the number of legions considered to be practical for a single general to command without becoming unwieldy and unusable?
It was my understanding that in a multi-legion battle there would be more than one general, but there would be one supreme commander who directed them.
In Republican times, the army consisted of either a 'consular' or 'double consular' ( if both Consuls were present). A consular army consisted of two legions of Romans (4,200 each aprox plus 300 or so cavalry) and an equivalent force of 'socii' Italian Allies.( sometimes larger than Roman legions) Each Legion, with light infantry velites skirmishing in front, was drawn up in three lines, and occupied a frontage of 200-250 yards aprox (180-230 m). The two Roman Legions occupied the centre with the 'socii' on each side, forming the wings ('Alae'). The cavalry formed up on each flank of the infantry.The total for a Consular army was around 20,000 or so.

The whole thus occupied 1,000 yards or so for the Infantry, and a further 250 yards or so on each flank for the cavalry. A double consular army could occupy twice the length.....

At Cannae an extraodinarily large force of 8 strong Legions ( 40,000 infantry) plus equivalent 'Socii' to make up a total of 80,000 Infantry and 6,000 or so cavalry was put in the field agaist Hannibal..
This force was drawn up extra deep, and the whole Roman force, Horse and Foot occupied a frontage in excess of 3,000 metres, with the Infantry occupying around half that, or a little more.

In Imperial times, despite a completely different organisation, similar sized forces were fielded. For example at Idistaviso, Germanicus fielded the best part of 8 Legions plus Praetorians totalling in excess of 29,000 Infantry, plus 30,000 auxiliaries, 8,000 cavalry and around 5,000 Germanic tribesmen. These were drawn up between a forest and a river in three unequal length lines with the front line around 1,500 metres long with the cavalry extended into the woods.

At the other end of the spectrum for a major battle, Seutonius Paullinus force consisted of the equivalent of one strong Legion (5-6,000( plus 4,00 or so auxiliaries and just two'alae' of cavaly (1,000 aprox). These were likely drawn up on a frontage of just over a mile or so (2-2,500 m) - a deliberately chosen narrow position - against a larger force of Britons under Boudicca, probably numbering 20,000 or even many more.

We might therefore suggest that forces numbering as many as 20,000 men, or more men on a frontage of 2-3,000 m ( 1-2 miles) could have a single commander.....

This is probably as good a rule of thumb as any......
At the Battle of Mursa 351AD, Contantius II and the usurper Magnetius fielded a huge number of troops, a large number of which were legiones. The size of the forces can be judged at the combined number of casualties, claimed by most sources to be over 50,000.
The largest number of legions combined into a single army seems to be Antony's army for the invasion of Parthia during his Triumvirate with Octavian and Lepidus. He gathered 16 of his 30 available legions along with 10000 Gallic and Spanish cavalry, 30000 auxiliaries including 16000 Armenian cataphracts the contingents from other vassal kings. The force was said to have totaled around 100000. Two of the 16 legions were detailed to guard the siege/supply train and were destroyed by the Parthians. The legions were understrength and all 16 together probably numbered around 60000. Even after detaching the two legions the force exceeded the strength of the Roman Army at Cannae. Unfortunately little is known of its battles except for a failed siege (due to the loss of the siege train) and an epic retreat with a number of rear guard actions. The Roman army Tiberius commanded in the suppression of the Illyrian Great Rebellion also numbered around 100000 but consisted of fewer legions, ten, along with 70 cohorts and 10 alae of auxiliaries but apparently this did not act as a single force and may have been divided into as many as three armies.
the largest Roman army reported is 188.000 - the army of Maxentius, 312 AD (Zosimus II.15.2). But Zosimus is always mentioning high numbers, plus we don't know how many of these were regular troops, and how many were hired federates, just for this campaign.

Quote:At the Battle of Mursa 351AD, Contantius II and the usurper Magnetius fielded a huge number of troops, a large number of which were legiones. The size of the forces can be judged at the combined number of casualties, claimed by most sources to be over 50,000.
Magnentius.
Were they legions or hired federates? Do we even know the exact number of men on the field, or just the number of casualties?
Quote:the largest Roman army reported is 188.000 - the army of Maxentius, 312 AD (Zosimus II.15.2). But Zosimus is always mentioning high numbers, plus we don't know how many of these were regular troops, and how many were hired federates, just for this campaign.

ValentinianVictrix:2muwa1r4 Wrote:At the Battle of Mursa 351AD, Contantius II and the usurper Magnetius fielded a huge number of troops, a large number of which were legiones. The size of the forces can be judged at the combined number of casualties, claimed by most sources to be over 50,000.
Magnentius.
Were they legions or hired federates? Do we even know the exact number of men on the field, or just the number of casualties?

Constantius II's army would probably have consisted soley of 'Roman' troops, Magnentius' forces were a mixture of regular Roman forces and Franks/Burgundian barbarians. Unfortunately the history this would have given the kind of information we needed about this, Ammianus' Res Gestae, has books 1-13 missing, and its book 13 that deals with this specific incident. 50,000 was the quote given for the number of casualties caused, not for the number of participants. Based on this figure both armies must have been quite substantial.
When asking what battle had the largest number of legions we must also take into account that the legions of the late empire only had around 1000-1200 men so a battle of Julian's era might have more legions involved than one of Augustus' time but far less men.
I wrote too soon that Antony's Army for the invasion of Parthia was the most legions gathered for a single campaign. At Philippi the Triumvirates had 19 full strength legions while Brutus & Cassius had 17 understrength legions. Counting both sides this is likely the largest number of the classic (5000 strong type) legions in a single battle. No doubt there were 3rd or 4th century battles with a greater number of the 1000 strong legions but potentially would have fielded smaller armies.

Also I misread the information about Tiberius' suppression of the Pannonian revolt. His ten legions were for a time massed into a single force comparable in size (counting auxiliaries) to Antony's.