This, of course, required the Romans to keep conquering new land so they could keep giving it to new legionnaires. Marius was also consul five times in a row, 60 years before Caesar. Or look at the Roman general Sulla, who, like Marius, ensured that his armies would be more loyal to him personally than to Rome, but who marched against Rome itself and then became its dictator, executing thousands of people in 81 BCE, 30 years before Caesar entered the scene. There's another way of looking at this question altogether if we dispense with great man history. Maybe Rome became an empire before it had an emperor. Like remember the Persian Empire? You'll recall that that empire had some characteristics that made it imperial, like a unified system of government, continual military expansion, and a diversity of subject peoples.