During the Social War, one of Marius' clients and friends, Manius Aquillius, had apparently encouraged the kingdoms of Nicomedia and Bithynia to invade Pontus.
In response, King Mithridates of Pontus responded by invading both kingdoms and Roman holdings in Asia (modern day western Asia minor).
Defeating the meagre forces at Aquillius' disposal, Mithridates marched across the Bosphorus and Aquillius retreated to Lesbos.
With the Social War concluded and with the prospects of a glorious and fabulously rich conquest, there was significant competition in the consular elections for 88 BC.
Eventually, Lucius Cornelius Sulla was elected consul, and received command of the army being sent to Pontus.

After Sulla left Rome for his army in Nola to depart for the East, Publius Sulpicius Rufus called an assembly which appointed Marius – a private citizen lacking any office in the Republic – to the command in Pontus.
Upon hearing of this, Sulla rallied his troops to his personal banner and called upon them to defend him against the insults of the Marian faction.
Loyal to Sulla and worried that they would be kept in Italy while Marius raised troops from his own veterans, the armies pledged their loyalty to Sulla.
Marius' faction sent two tribunes to Sulla's legions in eastern Italy, who were promptly murdered by Sulla's troops.

Sulla then ordered his troops to begin a slow march on Rome.
This was a momentous event, and was unforeseen by Marius, as no Roman army had ever marched upon Rome: it was forbidden by law and ancient tradition.

Once it became obvious that Sulla was going to defy the law and seize Rome by force, Marius attempted to organize a defense of the city using gladiators.
Unsurprisingly Marius' ad-hoc force was no match for Sulla's legions.
Marius was defeated and fled Rome.
Marius narrowly escaped capture and death on several occasions and eventually found safety with his veterans in Africa.
Sulla and his supporters in the Senate proscribed twelve men, including a death sentence passed on Marius, Sulpicius and a few other allies.
A few men – including Sulpicius – were executed but, according to Plutarch, many Romans disapproved of Sulla's actions.

Some who opposed Sulla were elected to office in 87 BC – Gnaeus Octavius, a supporter of Sulla, and Lucius Cornelius Cinna, a supporter of Marius and member of Sulla's extended family, were elected consuls – as Sulla wanted to demonstrate his republican bona fides.
Regardless, Sulla was again confirmed as the commander of the campaign against Mithridates, so he took his legions out of Rome and marched east to the war.
During the Social War, one of Marius' clients and friends, Manius Aquillius, had apparently encouraged the kingdoms of Nicomedia and Bithynia to invade Pontus. In response, King Mithridates of Pontus responded by invading both kingdoms and Roman holdings in Asia (modern day western Asia minor). Defeating the meagre forces at Aquillius' disposal, Mithridates marched across the Bosphorus and Aquillius retreated to Lesbos. With the Social War concluded and with the prospects of a glorious and fabulously rich conquest, there was significant competition in the consular elections for 88 BC. Eventually, Lucius Cornelius Sulla was elected consul, and received command of the army being sent to Pontus.
After Sulla left Rome for his army in Nola to depart for the East, Publius Sulpicius Rufus called an assembly which appointed Marius – a private citizen lacking any office in the Republic – to the command in Pontus. Upon hearing of this, Sulla rallied his troops to his personal banner and called upon them to defend him against the insults of the Marian faction. Loyal to Sulla and worried that they would be kept in Italy while Marius raised troops from his own veterans, the armies pledged their loyalty to Sulla. Marius' faction sent two tribunes to Sulla's legions in eastern Italy, who were promptly murdered by Sulla's troops.
Sulla then ordered his troops to begin a slow march on Rome. This was a momentous event, and was unforeseen by Marius, as no Roman army had ever marched upon Rome: it was forbidden by law and ancient tradition.
Once it became obvious that Sulla was going to defy the law and seize Rome by force, Marius attempted to organize a defense of the city using gladiators. Unsurprisingly Marius' ad-hoc force was no match for Sulla's legions. Marius was defeated and fled Rome. Marius narrowly escaped capture and death on several occasions and eventually found safety with his veterans in Africa. Sulla and his supporters in the Senate proscribed twelve men, including a death sentence passed on Marius, Sulpicius and a few other allies. A few men – including Sulpicius – were executed but, according to Plutarch, many Romans disapproved of Sulla's actions.
Some who opposed Sulla were elected to office in 87 BC – Gnaeus Octavius, a supporter of Sulla, and Lucius Cornelius Cinna, a supporter of Marius and member of Sulla's extended family, were elected consuls – as Sulla wanted to demonstrate his republican bona fides. Regardless, Sulla was again confirmed as the commander of the campaign against Mithridates, so he took his legions out of Rome and marched east to the war.
Gaius Marius