Timeline: Orbis Romanum
June 5, 44BCE Parthia
Julius Caesar sets out to avenge the defeat of Crassus against Parthia in 53 BCE where seven legions were lost. He sends one depleted legion from Syria to the mountainous regions of Armenia as a feint and two weeks later sets out in the direction of Ctesiphon with four legions, auxiliaries and 4000 cavalry from Gaul. A large engineering force is included for operating artillery and building defenses. The army is 40,000 strong overall.
Ctesiphon is a Parthian capital city 500 km from the Syrian border. It is larger and richer than Rome itself. An ominous sign for the Parthians is the presence of legion X Equestris under the command of Publius Crassus.
July 19, 44BCE Ctesiphon
The ruse is successful and Caesar arrives at the capital city of Ctesiphon in two weeks, outmaneuvering Parthian king Orodes II. The Romans build a fortress near the walls as is the custom if the city is too large to surround. Siege engines are dragged in position and start hammering at the walls.
King Orordes II arrives to relieve Ctesiphon with 2,000 cataphracts and 8000 mounted archers. He has good reason to believe he can beat Caesar the same way the larger army commanded by Crassus was beaten. The redoubtable Parthian archers on their armored horses are justly proud of their laminated bows with a pulling strength of 50 kg and capable of piercing armor. The Romans are well protected in their fortress however and sustain no casualties.
Parthian cataphracts watch expectantly from a safe distance. They will be used to break infantry ranks when the Romans decide to give battle.
To the consternation of the Persians they are bombarded with ballista volleys, not very accurate but lethal when on target. The Parthian army retreats and Caesar is satisfied. He has observed the enemy and everything is going according to plan.
July 20, 44BCE Ctesiphon
Early in the morning Caesar executes his plan. Gaul cavalry rides out and challenges Parthian archers. They wear no armor, unlike the heavily armored archers. They start playing a dangerous cat-and-mouse game, threatening attack then withdrawing as soon as the archers are in range. King Orodes II sends half his archers to give chase and both parties disappear over the horizon so that only a dust cloud can be seen. In the evening the archers set up camp as they intensely dislike night fighting. They are about 30 km away from the Roman fortress. Some tired sentries are appointed even though this appears unnecessary. Now the second phase of Caesar's plan unfolds. A Gaul spy who has been shadowing the Parthians arrives at the Roman fortress. He leads a party of veteran light auxiliaries to the Parthian camp in a forced march. The Parthian sentries are exhausted and many have fallen asleep on the job. The auxiliaries wait while one by one the Parthian lookouts are overcome by chosen assassins, shedding the first blood. An owl hoots three times. The auxiliaries silently close in on the sleeping Parthians. A Parthian with a weak bladder wakes up to relieve himself and notices furtive shapes in the dark. A cry goes out but too late. With one accord and a great shout the auxiliaries attack. The archers are exceedingly tired and at a disadvantage in the dark. They are cut them down mercilessly. Within two minutes most are dead or wounded. 250 manage to escape on foot while the auxiliaries dispatch the wounded. King Orodes II has lost almost half of his army.
July 25, 44BCE Orodes II
In the morning Roman legions line up outside the fortress. This was what king Orodes II wanted all along. The infantry advances in testudo formation as protection against archers. This makes them vulnerable to cavalry attack as king Orodes II well knows.
Excited by an obvious mistake by Caesar he orders a charge by the cataphracts. The cavalry thunders forward aiming to cut the Roman army in two. At that moment Roman engineers remove camouflage nets from concealed ballista positions installed during darkness. Fast firing ballistae let fly at the cataphracts at optimum range and cause pandemonium. The cries of horses and men mingle as the Parthians are cut down row by row. The enemy ranks break and the cataphracts are routed, pursued by a last volley from the ballistae.
Caesar gives a sign to the Gaul cavalry. They have orders to bypass the Parthian lines and cut off the retreat. The remaining 4000 Parthian archers are not aware of the danger and keep firing at the Roman testudo formations. Their attention is also distracted by ballistae that are being dragged into new firing positions. Then a cry of alarm reverberates along the ranks as the Gaul cavalry charges from the rear. The archers give a good account of themselves at first against the unarmored Gauls but then the legions charge into the fray and it is all over minutes. King Orodes II is captured in his wagon and dragged before Caesar, who is well aware that this is a historic moment. He says "While the Gods were looking the other way a victory fell into your lap and you captured Roman Aquilae. I want them back. Your life for the aquilae." Orodes II agrees readily. Caesar is known as an honorable man even to his enemies.
The siege continues and late that day the first cohort of X Equestris breaks through the walls of the city.
July 26, 44CE Ctesiphon
The city has surrendered to Caesar. Surviving defenders and a large number of women and children will be sold in slavery, the customary punishment for thosewho dare oppose Rome. King Orodes II will feature in Caesar's triumph in Rome. Octavian is left in charge in Ctesiphon.