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He established a professional army, including instituting for the first time the use of the weapon known as the pilum - a thrown spear designed to shatter upon impact so that the enemy could not re-use it after thrown by a Roman legionnaire.
He also established the army as an institution more related to the individual general than the state, by for the first time personally rewarding his veterans. In this sense, Marius is said to have "forged the sword that slew the state," in that he created the idea of a personal army, which Gnaeus Pompeius Magnus and Julius Caesar would later use in the civil wars that led to the fall of the Roman Republic.
After leading a successful military career, Marius, instead of retiring, attempted to become engaged in Roman political life by joining in the conflict between Lucius Cornelius Sulla and the Roman Senate. The Greek historian Plutarch tells us that Marius was incredibly inept at politics, and uses his life in the book Parallel Lives as an example of what happens when a man stretches himself too far, and refuses to be content with his already-memorable legacy.
Plutarch, Parallel Lives, "Life of Caius Marius."