He took part in the Social War in Rome
Chaos erupted in Rome in 91 BC following a brief period of peace following the murder of the court's Marcus Livius Drusus. This sparked a revolt in Rome and what came to be known as the Social War. A Roman army under the command of Gaius Marius was ordered to battle the insurgents. He achieved this, but he only made it through one campaign before giving up.
He got into a fight with the Roman general Sulla, and as a result, he was banished to Africa in 88 BC. During the Octavius War, Marius returned to Italy, invaded Rome, and unleashed a brutal reign of terror that culminated in his election as consul for the seventh time and his subsequent demise at the start of his term, in 86 BC.
Marius had withdrawn or left the battle by 89 BC. Either he genuinely was ill, or he retreated and claimed to be unwell because he felt underappreciated. It's also possible that the government did not extend his order when it expired in late 90 BC because it had failed, or that they gave him a life-saving deal: retire, leave, and get medical care.
His life and work helped Rome become an empire by upending the several conventions that held the aspiring elite of the Roman republic together and by founding a faction that was devoted to its leaders rather than the republic.
The reason for his retirement is said to be that he had health issues, but many academics think it was a political decision because Marius was under a lot of pressure to treat his main opponents at the time.
