The Road to War: A Timeline of Caesar and Pompey's Breakup
All dates should be treated as tentative. Items in red raise hostilities. Items in blue are friendly or conciliatory.
The biggest ruptures appear to have occurred when Curio began his "persistent veto" in March of 50, and advocated for Caesar's interests while attacking Pompey; Caesar's refusal to run for election the following summer as Pompey expected; and C. Marcellus' attempt to initiate war against Caesar at the end of that year.
55 BCE
Pompey and Crassus' second joint consulship.
Caesar's Gallic command is renewed "for five years"; precise termination date is either unclear or unnamed.
54
Crassus invades Parthia.
Death of Julia (Caesar's daughter and Pompey's wife).
Caesar supports Gabinius, Pompey's former legate, while he's on trial.
Pompey lends Caesar an additional legion for Gaul.
53
Caesar proposes a new marriage arrangement to Pompey, who declines. However, Pompey remains Caesar's primary heir in his will. This probably did not significantly damage their alliance.
Death of Crassus at Carrhae.
52
Death of Publius Clodius Pulcher. Breakdown in public order.
In lieu of elections, Pompey is appointed sole consul.
Caesar declines being appointed co-consul because he's busy with Vercingetorix's revolt in Gaul. Instead, he requests a ratio absentis, the right to be allowed to run for office in absentia, to ensure he won't have to abandon Gaul mid-war. The bill, supported by all ten tribunes, is passed in the People's Assembly.
Pompey marries Cornelia, daughter of Metellus Scipio (an optimate), and appoints Metellus Scipio as his co-consul.
Pompey enacts a law affirming that all candidates for office must canvass in person, Caesar excepted. In another law Pompey alters allocation of provinces, excepting his own and Caesar's extraordinary commands.
51
Caesar wraps up the Gallic campaign. Most senators expect him to run for the consulship in 50.
Spring
Consul M. Marcellus tries to prematurely recall Caesar from Gaul in this year, and to remove his ratio absentis. Discussion is continually delayed by lack of support for Marcellus' measure in the Senate.
Summer
Marcellus assaults a Roman citizen of Transpadane Gaul as an insult to Caesar and Pompey. Catonians attempt to drive a wedge between Caesar and Pompey by asking why Pompey has not demanded his legion back from Caesar; Pompey deflects the issue.
One of Pompey's allies is seen moving to join Caesar after being convicted in a trial, indicating continued alliance. Election strategies are still planned with the assumption of Caesar and Pompey working as a team.
Autumn
Pompey plans to move to Syria to defend against a potential Parthian invasion, which indicates he wasn't afraid of Caesar invading at this time.
Pompey flatly rejects a motion by M. Marcellus to terminate Caesar's command of Gaul in March.
September 30 - Pompey and the Senate postpone discussion of Caesar's recall until March of 50. Caesar (or his tribunes) permit this bill to go through, apparently unconcerned.
50 - The year where everything goes to shit
February
Curio the tribune makes an abrupt shift to supporting Caesar, while attacking Pompey. Caesar may have sought additional allies (like Curio) in response to Pompey reconnecting with some of Caesar's enemies, such as Metellus Scipio.
March
Consul C. Marcellus again tries to recall Caesar from Gaul. Pompey permits discussion of the recall (in accord with the previous bill passed in September). Curio vetoes, and sustains said veto throughout 50. It's unclear if Curio was acting on his own initiative or Caesar's, but his persistent veto raised fears that Caesar would refuse to respect the Senate's authority. Curio could not have sustained a persistent veto without significant popular support, or else there would've been a recall election. Curio and Caesar's alliance with Aemilius Paulus may have further rattled Pompey.
April
The Senate and Pompey attempt to compromise with Caesar by acknowledging his right to run in absentia for the summer of 50, but require him to stand down from Transalpine Gaul by November. Curio vetoes, claiming to speak for Caesar (which not everyone believes) and attacking Pompey.
The Senate requires Caesar and Pompey to each contribute one legion to the defense of Syria. Pompey requests back a legion he had "loaned" to Caesar, thus removing two legions of Caesar's forces and costing Pompey none. When the Parthian threat dissipates, the legions are not sent to Syria but quartered in Italy. This probably caused Caesarians to feel betrayed and concerned of a Pompeian military buildup.
April-July
At some point Curio proposes that Caesar and Pompey should both step down from their military commands. This may have been his (or Caesar's) attempt to de-escalate the situation; it's likely Caesar supported the bill. Pompey rejects it, on the grounds that Caesar's command had expired last March, while Pompey's was still in effect.
May-June
C. Marcellus proposes a bill to severely punish Curio for his veto, claiming it was inimical to the interests of the state. No such law had been passed, but by attacking Curio's veto Marcellus also hoped to destroy Caesar's claim to an extension of his ratio absentis. Pompey is forced to go along with a rejection of the extension or else be publicly embarrassed by contradicting his earlier promises/announcements. However, the Senate as a whole refuses to back the bill, preferring a peaceful resolution. This implicitly grants a postponement of the provincial reassignment and for Caesar to run in 49 in absentia.
Pompey has a serious illness at some point in this period.
June
Pompey agrees to Curio's mutual disarmament proposal; Curio insists that Pompey disarm first, and Pompey feels insulted and rejects that. Distrust increases on both sides.
July/August
Caesar declines to run for the consulship in absentia, for reasons that aren't clear. He may have expected his candidacy to be blocked on spurious grounds, or for the election to be rigged. Or he may simply have needed more time to set up provincial administration in Gaul.
He sends his legate Galba to run instead. Galba loses the election, possibly due to fraud (according to Hirtius). This exacerbates both Caesar's distrust that he could have a fair election without an army to support him, and senatorial fears that Caesar planned to hold onto his army indefinitely.
After this point M. Caelius Rufus first mentions fears of a civil war.
Autumn
Caesar raises four additional legions. This may have been intended defensively in response to Pompey taking two of his legions, or as saber-rattling to intimidate the Senate, but either way it increases people's fears.
November
Caesar returns to Cisalpine Gaul; he refuses to step down from his command at the date decreed by the Senate. At some point Caesar summons the 8th and 12th legions to Cisalpine Gaul as well, though he claims he only did so in January.
December
December 1-2? - C. Marcellus holds a vote on whether Caesar should step down (Senate votes yes), whether Pompey should step down (Senate votes no), and Curio then has them vote on whether both should step down (94% YES). The people in the forum hail Curio as a hero and expect mutual disarmament to occur. Marcellus dismisses the Senate before the bill can be passed.
False rumor spreads of Caesar invading Italy. Consul C. Marcellus perpetuates it, while Curio rejects it. Marcellus attempts to have Caesar declared a public enemy and to have the two legions sent to fight him. Curio vetoes. Marcellus ignores the veto and personally asks Pompey to take command of the legions and "defend the state." Most observers interpret this "Schwertübergabe" (sword-handover) as a military action against Caesar, or even an outright declaration of war.
December 6 - Caesar's envoy Hirtius visits Rome but doesn't try to meet with Pompey or Metellus Scipio, which they take as an insult. It's unknown whether Caesar sent Hirtius in response to the Schwertübergabe or if Hirtius altered his plans upon hearing of it.
December 7 - Pompey moves south to the "Parthian" legions, not north to face Caesar. This probably indicates that he and Marcellus did not believe Caesar was actually invading Italy as Marcellus claimed.
December 9 - Curio's term as tribune ends, and he immediately leaves to join Caesar in Gaul, perhaps fearing for his safety. He's replaced by Mark Antony, who continues the persistent veto, and whose attacks on Pompey further raise tensions.
December 25 - Pompey tells Cicero he neither hopes for nor desires peace anymore.
49
January 1 - Caesar reiterates to the Senate (via letter) that he will step down from his command only if Pompey does (a reiteration of Curio's disarmament proposal), and if Pompey doesn't, Caesar will "come with all speed to defend himself and the Republic." The anti-Caesarian consuls refuse to let Caesar's letters be read aloud in the Senate, in the belief that this would make more people side with Caesar. (I'm unsure if this is one incident or two.)
January 4 - Cicero returns to Rome, and describes a "war frenzy" among a small group of senators, in opposition to an overwhelming desire for peace among most of the Senate and People.
January 6 - Caesar offers a peace deal via Curio: to step down from command of both Gauls (leaving only Illyria), and all but one of his legions. Pompey nearly accepts the offer before C. Marcellus, Cato, and Lentulus Crus dissuade him. Negotiations fall apart.
January 7 - The senatus consultum ultimum is passed against Caesar, and a state of tumult is declared in Italy. Caesar's tribunes are forced out of the Senate (and perhaps physically attacked), and leave to join him in Cisalpine Gaul.
January 10/11 - Caesar crosses the Rubicon.
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Sources: The Last Generation of the Roman Republic by Erich Gruen, and Julius Caesar and the Roman People by Robert Morstein-Marx.