Cicero dies of old age and Publius Quintus Copellus is voted in as Princeps Senatus for a 10 year term.
In May, Caesar arrives in Egypt along with Cleopatra and his army from the German campaign and takes charge of the huge army she and her allies in Egypt have prepared for him for years. He takes charge. Caesar's army, which he recruited from veterans of his campaigns and new recruits a year earlier, number nearly 150,000 and the army in Egypt is 200,000. As promised, Scipio is with him and Caesar gives him command of the Egyptian army with his subordinate being Pharxes. In September, Caesar positions his and Scipio’s army in Syria. Caesar and Scipio invade the Parthian empire. The first weeks go well for Caesar and Scipio as they meet a small Parthian army of 10,000 and easily crush it between their two massive armies at the Battle of the Araxes River. King Phraates of the Parthians begins to call up a large army of 350,000. In December, Caesar meets a 220,000 man Parthian army and fights a series of skirmishes along the Euphrates. After 3 inconclusive battles along the southern Euphrates and heavy casualties on both sides, the Parthian king withdraws across the Euphrates, destroying all the bridges in the process. Caesar wants to build a bridge and confront the Parthian king in a deciding battle, but the Parthians quickly build defenses along the river. Caesar marches his army south to cross at a different point, but the Parthian army follows his army south along the river. Caesar sends a message to Scipio (who is busy further north, having just captured Ctesiphon) to quickly march to the Tigris, cross the river and burn the bridge, and fall on Phraates from the north. Caesar continues to distract Phraates, and the two armies frequently launch stones and arrows at each other.
Antony campaigns in the Caucasus Mountains, subduing several tribes and securing the Roman border in the east. He continues to build defenses in the mountains on his march south toward Armenia.
Scipio encounters several small Parthian armies and crushes them rather easy at the Battle of Ctesiphon in early December, capturing the vital city in the process. Scipio proves to be a superb tactician by outflanking his enemies multiple times in one battle and even uses a feigned retreat to lure a Parthian force of 17,000 into a trap at the Battle of Artemita in late December. Scipio uses his excellent diplomacy skills to convince some to the Parthian soldiers to switch sides. He offers them twice the wage the Parthian king is paying them and free land after the conquest of Parthia. Many accept, and by March 32 B.C., he has gained an additional 120,000 troops. One of the most famous instances of treachery (according to Livy) among the Parthian generals was Gondophares, who switched loyalties to the Roman army after Scipio offered him a huge bribe in exchange for his services and large army of 80,000 elite Parthian soldiers, consisting largely of cataphract cavalry.
In October, Agrippa invades the Parthian provinces in Asia (OTL modern Turkey), defeating the Parthian army at the Second Battle of Carrhae, utterly crushing the Parthian general Orthodus and regaining the standards of Crassus. One reason for his overwhelming victory and similarly, that of Scipio (according to historians) is the extensive use of heavy cavalry by the Romans for the first time in Roman military history.
By the end of 33 B.C. most of Mesopotamia (including the Euphrates and the western bank of the Tigris) are in Roman hands along with several provinces in Asia and the Parthian capital Ctesiphon. The new Parthian capital becomes Persepolis in Persia.