The Grass Crown (Masters of Rome #2) by Colleen McCullough (1991, First edition)

In this great drama, Marius, the general who saved Rome from barbarian invasion and became consul an unprecedented six times, has fallen into decline. Sulla, his closest associate, has withdrawn himself from his commander's circle in preparation for his own bid for power. As a deadly enmity develops between the two men, Rome must fight its own battle for survival - first against her neighbouring Italian states, then against the barbaric Asian conqueror. Births, deaths, prophecies and rivalries combine to create a whirlwind of drama, and a remarkable insight into the passion and torment of ancient Rome.