Why did civil wars occur in the Rome during the first century?

Rome was facing a rebellion in Asia Minor that required Rome intervention. The people wanted Marius to lead the war but the senate chose Sulla, who had become a consul. As a result Sulla marched on Rome and declared Marius an outlaw. Marius left to Africa, and Sulla went to the east to deal with the rebellion. As soon as Sulla left for Asia, Marius and his supporters went to take control of Rome once again. Marius died in 86 BCE but his followers continued the battle to take control of Rome. In 82 B.C.E., on his way back from the east, Sulla marched in yet again with his army and the first civil war broke out.

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