With the Traianic Roman army invading, the Parthians deployed a small local army of a local Satrap to relieve the siege of one of their cities on the Armenian border. A serious defeat would probably have cost him the rest of his satrapy as other contenders abounded. So, very carefully, he set out to recon the huge enemy force.
The first surprise was when an outflanking force of some 2000 horse archers (Irr LH O, F, (Pro), Msl [Bow]) sent northwest, to sweep around the Roman column, ended up being the main force as it unexpectedly met the right flank of the fully deployed Roman Army on its way. The main Parthian force of another 3000 horse archers and 1800 Cataphracts (Irr Cat O, Shk) finally showed up on the other flank of the Roman Army and the battle was never going to be a boring one.
The Romans under Traianus had over 40,000 men with a well-balanced force of three + legions (Reg Sw O, Pro, H, Shk/Msl [Pila]), plenty of Auxiliarii (Reg Plt O, Pro, H, Shk/Msl [Jav]), mobile artillery in support (also Pro), light troops and some 5000 heavy javelin-armed cavalry and 2000 light cavalry. They also had a significant contingent of 6000 Armenian allies with heavy and light infantry and over 2000 Cataphracts in support. Overall, there was safety in numbers and quality.
As Traianus did not trust the Armenians he had them sandwiched between his main force of two legions in the center, with artillery, and a reserve force of one full legion in their rear, with their backs to the city under siege. The flanks were to be protected by the Auxilia and cavalry plus numerous skirmishers.
The first light force of Parthian light cavalry arrived on table and immediately went for the Romans’ left flank to avoid the artillery in the centre. They were met by over 2000 light archers and slingers and an exchange ensued. However, their initial archery was below their usual standards and they lost as many men to the light missilemen as they inflicted on them.
Right then, the main Parthian force arrived on the extreme Roman right flank and the battle really heated up. The Roman commander was initially hampered in meeting the double envelopment due to poor communications and he decided to rely on the combination of Auxilia and cavalry to fight the light horse archers. It seems he had learned nothing from the painful Roman past… As the Parthian archery remained poor in the initial exchanges, the skirmish lines had remained almost intact and the Roman horsemen were well protected, but this gave them over-confidence which was to cost them dearly.
On the Roman left, the horse archers finally broke through the enemy light screens and, having ample room to manoeuvre, started isolating legionnaire and auxiliary detachments here and there with devastating results. The Romans were baffled. They forgot their artillery, which failed to fire a single bolt and went after the Parthian horse archers with groups of heavy bowmen with auxilia in support. When this failed, and seeing their poor results and mounting casualties, (over 1000 auxiliaries of all sorts and even half a legionnaire cohort) they charged out of the protective screens with their heavy horse. Some 2500 heavy horsemen attacked again and again and less than a third of their number made it back after these desperate - and failed - charges. The Parthians were simply too swift.
On the Roman right, the Parthians depended more on their Cataphracts whose strength, however, was not comparable to that of their ancestors’. They charged through the enemy light screens, unsupported, against the Auxilia and were soundly defeated after a hard fight of over half an hour. They lost a third of their number causing fewer casualties on the Romans and failed to break through. Cataphracts should never be used without archery support. The Parthian horse archers, however, were not entirely inactive and successively destroyed a full Auxilia cohort, most of the skirmishers, the few regular bowmen that tried to intervene and eventually most of the Roman cavalry, which made the same fatal mistake as their comrades on the other side of the battlefield. Their losses, except for the Cataphracts, were rather on the light side.
At that point, with darkness falling, the Romans started re-deploying their artillery and the Parthians decided that they had done enough and could do little more. 6000 Parthians had inflicted almost 8000 losses on an elite force of 40,000 Romans, but their own 1500 losses were too many for their tiny force. They had destroyed almost the entire Roman cavalry force, a large portion of their light infantry and caused serious losses to the Auxilia infantry and even the legionnaires. The Armenian allies of the Romans were really uneasy and had it not been for the third legion in reserve behind them things could have been disastrous. In the end, a foray of the besieged against the reserve legion was beaten back and the Romans remained masters of the battlefield. The city fell after two days, but the campaign was virtually over. The Romans had to pull back to replenish and reconstruct their crippled cavalry force.