.

In the standard roman legion deployment, the heavy infantry was deployed 3 lines deep with a wide gap between each line. Each line was made up of maniples/cohorts and the gaps between two adjacent maniple/cohort was about as wide as the formation itself. That is the traditionaly held view.

What I fail to understand though, is how did this hold up against enemies like the gauls who chargeden masse. Wouldn't they just surround the individual units of the first line and anahilate them before the second roman line could help them out?

This was more a manuvering formation than a battle one. After the Velites fired off their javilens they retreated thru the open Hastati formation. The Hastati closed into a single line before engaging the enemy. If the Hastati wavered, they would retreat thru the open principes lines, who would then form a single comat line
 
Roman infantry tactics, whether manipular or cohortal, were build around a number of key points:

First, the pilum was designed to break up enemy formations, whether charging Gauls or more stationary opponents being charged by the Romans, right before the moment of impact. Part of the pilum's design was that its head, after penetrating an opponents shield, would lodge itself there, forcing that warrior to either drop the shield or attempt to fight with one hand essentially tied behind his back. As your average Gaul wore no armor, loss of his shield left him with no protection.

Secondly, the Romans employed a large shield - the scutum - and a short stabbing sword - the gladius. This combination allowed them to maintain much tighter formations than those who relied on longer slashing blades as did the Gauls. A unit of Romans, in maniple or cohort, could thus stand shoulder to shoulder and still wield their weapons to full effect. If packed in as tightly, Gauls and the like would be to constricted too properly hack and chop.

Thirdly, the combination of scutum and gladius, along with Roman drill and discipline, enabled reserve units to directly relieve units already engaged by passing through their ranks in good order. So, as pawpaw mentioned, if the hastati were in danger of being overwhelmed, or simply needed a breather, the principes could filter through the ranks of the hastati to engage the enemy while the hastati withdrew in good order. The same applies to the reserve centuries of a cohortal legion reinforcing and relieving their frontline comrades.

Finally, a Roman legion rarely fought alone. Not only were there usually more than one legion operating in unison, they almost invariably went into battle with an assortment of light troops and cavalry. These light troops - which would have included the velites in the manipular legion, mercenary slingers such as the Balearics and archers such as the Cretans, and the various numerii and symmacharii light troops of the early empire - would deploy forward of the main battleline and fall back as they shot up the advancing enemy. A legion would also, in all likelihood, have deployed artillery in the form of ballistae or onagers to start killing and demoralizing the barbarous hordes before they even came within charge range.

The barbarians, on the other hand, lacked the tactical flexibility, and usually the morale, to drive home their initial charge in most cases. As the best and bravest warriors strove for a place in the front ranks, those behind were usually less skilled and/or less enthusiastic. Therefore, if the initial charge failed to carry the day, a barbarian horde would come apart as the front ranks died spitted upon Roman pila or gutted bu Roman gladii while the less committed fled.

Obviouslt, this didn't always play to script (Trebia,Lake Trasimene and Cannae, Teutoberger Wald, the Dacian War, Septimus Severus' failure to subdue the Caledonians among others). But it worked often enough to build and maintain the empire until the growing number of mobile, horse-based adversaries forced the Roman military to evolve in the early Third Century.
 
Trebia was different. Hannibal didn't charge en masse, he used flanking heavy cavalry and elephants to devastate the Roman line while his light infantry mopped up. Unless the Roman line was broken, it was often unstoppable.
 
Great overview, Miles.
A unit of Romans, in maniple or cohort, could thus stand shoulder to shoulder and still wield their weapons to full effect.
This is true, but IIRC typical Roman line infantry wanted a space of a few feet on each side to better wield his weapon. Although they definitely did work phalangially, in the later Empire especially, it was the flexibility provided by that more open order that helped the Romans infiltrate the Macedonian phalanx at Pydna and which generally served them well against a more tightly packed opponent.
MilesGregarius said:
Obviouslt, this didn't always play to script (Trebia,Lake Trasimene and Cannae, Teutoberger Wald, the Dacian War, Septimus Severus' failure to subdue the Caledonians among others).
And even then, a lot of those examples were tactical oddities; at the Trebia and Lake Trasimene, the legion was taken in flank and/or rear, in the Teutoberger Wald they were unable to properly deploy. The Caledonian conflicts were IMHO more of a strategic problem than a tactical one. And as for Cannae, the interesting thing is that the Roman legions that were surrounded by Hannibal's troops still managed to keep plowing forward, and more than ten thousand of them broke out through the center of his army and were able to flee (to Capua I believe, though it's been a while since I read Polybius). So again, the problem there was definitely the cavalry flank attacks and the excellent positioning of the Punic heavy infantry on the flanks as well.
Hannibal didn't charge en masse, he used flanking heavy cavalry and elephants to devastate the Roman line while his light infantry mopped up..
True, true, and I'd like to add that even then Hannibal never sent his cavalry straight at a legion, because that'd be suicide. Formed heavy infantry were able to repulse cavalry almost always, and needed to be taken in flank or rear for the cavalry to have full effect.
 
What I fail to understand though, is how did this hold up against enemies like the gauls who chargeden masse. Wouldn't they just surround the individual units of the first line and anahilate them before the second roman line could help them out?

The romans had large, sturdy shields, excellent armor, solid close-range weapons, and of course discipline. Against poorly organized barbarians, and with supplemental archers, spearmen, and cavalry, their lines were basically unstoppable.

Imagine this situation: You are a skilled Gaelic warrior with a simple round shield and a large, heavy sword. As you charge the Roman line, they unleash a volley a javelins; one lodges in your shield, the heavy spear bends and is caught in your shield. You are forced to drop it lest you get weighed down and pinned down by arrows. Using both hands (now that your shield hand is free), you bring your huge sword down into a legionary's wooden shield, lodging it partway in. As you pull it out, the enemy soldier stabs you in your unprotected belly.

The End
 
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