The pilum (plural pila) was a javelin commonly used by the Roman army in ancient times. It was generally about 2 metres (6 ft 7 in) long overall, consisting of an iron shank about 7 millimetres (0.28 in) in diameter and 60 centimetres (24 in) long with pyramidal head. The shank was joined to the wooded shaft by either a socket or a flat tang.
The total weight of a pilum was between 2 and 5 kilograms (4.4 and 11.0 lb), with the versions produced during the Empire being a bit lighter than those dating from the previous Republican era.
The iron shank was the key to the function of the pilum. The weapon had a hard pyramidal tip but the shank itself was usually not hardened. The softness of the shank would cause it to bend after impact, thus rendering the weapon useless to the enemy. However, there are many cases where the whole shank was hardened, making the pilum more suitable as a melee weapon. More importantly, If the pilum struck the shield of an enemy it would embed itself into the shield's fabric, and this along with the bending of the shank would cause the shield to become unwieldy, forcing the enemy to discard it or waste time trying to pull it out. The former action tended to happen more often than the latter as affected soldiers couldn't risk removing the pilum without disrupting their formation during an advance or losing their lives during a melee. Even if the shank did not bend due to the design the pyramidal tip still made it difficult to pull out of corpses, armour and shields.
Most javelins were unable to penetrate a shield. By contrast, since the pyramidal tip of a pilum was wider than the rest of the shank, once it penetrated a shield, it left behind a hole larger than the rest of the shank, and it could move through with little resistance, often stabbing the soldier bearing the shield. The length of the shank and its depth of penetration also made it hard to pull out of a shield even if it failed to bend. If the bearer of the shield was charging and a Pilum penetrated the shield, the end of the heavy shaft of the Pilum would hit the ground and dig into the ground, holding the shield in place while the charging soldier's momentum carried him forward. Often this would cause the soldier to skewer himself on the shaft of the Pilum sticking through his shield. On some pila there was a spike on the shaft either carved from the shaft or made from iron. This made it better for digging into the ground.
Pila were divided into two models: heavy and light. Pictorial evidence suggests that some versions of the weapon were weighted by a lead ball to increase penetrative power but archeological specimens of this design variant are not so far known. Recent experiments have shown pila to have a range of approximately 33 metres (100 ft), although the effective range is up to 15–20 m (50–70 ft).
An Angon was a similar weapon used in late Roman and post-Roman times.
Legionaries of the Late Republic and Early Empire often carried two pila, with one sometimes being lighter than the other. Standard tactics called for Roman soldiers to throw one of them (both if time permitted) at the enemy, just before charging to engage with the gladius, however Alexander Zhmodikov has argued that the Roman infantry could use pila at any stage in the fighting.
The effect of the pila throw was to disrupt the enemy formation by attrition and by causing gaps to appear in its protective shield wall.
Pila could also be used in hand-to-hand combat; one documented instance of this occurred at the Siege of Alesia, and another during Marcus Antonius's Parthian campaign.  Additionally, pila could be employed as a thrusting implement and a barrier against cavalry charges. Some pila had small hand-guards, to protect the wielder if he intended to use it as a melee weapon, but it does not appear that this was common.
As to the missile weapons of the infantry, they were javelins headed with a triangular sharp iron, eleven inches (279 mm) or a foot long, and were called piles. When once fixed in the shield it was impossible to draw them out, and when thrown with force and skill, they penetrated the cuirass without difficulty.
And later in the same work:
They had likewise two other javelins, the largest of which was composed of a staff five feet and a half long and a triangular head of iron nine inches [230 mm] long. This was formerly called the pilum, but now it is known by the name of spiculum. The soldiers were particularly exercised in the use of this weapon, because when thrown with force and skill it often penetrated the shields of the foot and the cuirasses of the horse.
It may be argued that a short iron shaft has very few confirmations from archeology. Vegetius is writing about a one foot iron shaft because at Vegetius' time the pilum had disappeared and been replaced by similar shorter weapons such as the plumbatae or the above mentioned spiculum.
Results of experimental archaeology
Thanks in part to experimental archaeology, it is generally believed that the pilum's design evolved to be armour-piercing: the pyramidal head would punch a small hole through an enemy shield allowing the thin shank to pass through and penetrate a distance sufficient to hit the man behind it. The thick wooden shaft provided the weight behind the punch.
In one description, one of the two iron nails that held the iron shaft in place was replaced with a weak wooden pin that would break on impact causing the shaft to twist sideways. Gaius Marius is sometimes given credit for this modification. It has been argued that later pila were constructed so that the iron shank would bend on impact. Having penetrated a shield through a small hole and its shank having bent, such a pilum would now be more difficult to remove, and, of course, impossible to throw back.
Further complications and injury could ensue if the understandably-reluctant enemy did not discard his precious shield quickly enough, as there would be a great press from the men behind him.
Opinion among archaeologists once held that the main function of the shank was to disable both shields and the pilum itself by bending, but it is now thought that the pilum was a form of "personal artillery" designed simply to provide a massive counter-shock against any charging foe, and, as necessary, turn any legionary into a spearman.
- Pole weapon
- Roman military personal equipment
A Roman coin showing an antoninianus of Carinus holding pilum and globe.
- Cowan, Ross (2003). "Equipment". Roman legionary: 58 BC - AD 69. Osprey Publishing. pp. 25–26. ISBN 978-1-84176-600-3. Retrieved 8 February 2012.
- Connolly, 1998, p233.
- Zhmodikov, Alexander, 2000, "Roman Republican Heavy Infantrymen in Battle (IV-II Centuries B.C.)," in Historia: Zeitschrift für Alte Geschichte, vol. 49 no. 1.
- "tois yssois paiontes." Plutarch. "Life of Mark Antony, 45.3". Retrieved 2011-09-27.
- Arrian's Array against the Alans. "And the front four ranks of the formation must be of spearmen, whose spearpoints end in thin iron shanks. And the foremost of them should hold them at the ready, in order that when the enemies come near them, they can thrust the iron points of the spears at the breast of the horses in particular. Those standing in second, third and fourth rank of the formation must hold their spears ready for thrusting if possible, wounding the horses and killing the horsemen and put the rider out of action with the spear stuck in their heavy body armour and the iron point bent because of the softness. The following ranks should be of the javelineers."Dorst, Sander van. "Arrian's Array against the Alans". Retrieved 2010-10-03.
- Vegetius. "Book I". De Re Militari. Retrieved 2006-08-24.
- Vegetius. "Book II". De Re Militari. Retrieved 2006-08-24.
- Plutarch, "Gaius Marius", 25.2
- Connolly, Peter. Greece and Rome at War. Reprint: Greenhill Books, 1998 ISBN 1-85367-303-X.
- Connolly, Peter. "The pilum from Marius to Nero: a reconsideration of its development and function", Journal of Roman Military Equipment Studies, vol. 12/13, 2001/2, pp. 1-8.