I think its the Roman Republic. They based all their wars on the idea that if they aren't first to strike, they would be first struck and that it was necessary to keep subjugating their neighbors to ensure their existence. The Republic's capital Roma was sacked by the Gauls and after they reformed they went on a spree of war and conquest. The than most powerful group of nations in Europe the Greeks than answered the pleas of the Greek settlements in southern Italy (Tarentum, Croton) and Pyhrrus of Epirus took on the task of suppressing the Roman tribes power gobble. But that failed and even though he won the battle, the Romans inflicted such losses to his ranks he was unable to contend with the Romans (or the Carthaginians for that matter).
"What a battlefield I am leaving for Carthage and Rome" - Pyhrrus of Epirus is quoted saying. I'm sure I don't need to go into all the details of the Punic Wars, the invasion of Macedonia and Greece, the war with Selecuia or the gauls, germania, iberia, britons, or thracians. Long story short, Rome gobbled up all of its neighbors in a rapid amount of time (something not outdone until the Mongols hordes sweeped across Eurasia, except they conquered more in a fraction of the time due to their methods). The Roman Legions - the early Republic armies (early hoplie/phalanx, polybian, and Marian reformed Legions) were constantly growing to ever new standards of proffesional, organized and skillfull levels. They had solid systems for engagement, successful training regimes, and a very mobile and flexible army (which proved to defeat all its adversaries - from the Phalanx-Eastern Armies of Greece and those who were influenced by it and the barbarian tribal armies of various styles). Their armies must've been a truely intimidating sight to any army going up against it.
Also, a look at the Roman culture yields alot of cruelty. They were cruel in many instances, their woman were caniving and active in politics of the men from the sidelines (example being Cornelia and Fulvia), the people enjoyed bloodshed and killings in the arenas. The crusifying of people is also a cruel manner. The politics were relentless, cutthroat and fierce during the last days of the Republic resulting in two civil wars on a large scale, mass killings of political enemies, and suffering for many Romans. Overall, I feel anyone living in the time of Rome who was not a Roman citizen would honestly be scared **** of them. Both as a nation and as a people. I know I wouldn't want to be on the bad side of a nation with so much power and so little regard for human life lol.