Hannibal Barca, one of the preeminent military minds of his age, nearly managed to destroy the Roman Empire. Using a daring traversal of the Alps with his War Elephants, Hannibal reached the gates of Rome before lack of supply and reinforcement made him retreat. After the fall of Carthage, he fled to modern-day Turkey, a guest of monarchs and a living symbol of Roman vulnerability. 

The Revolt of the Ionian Greek cities (in modern day Turkey) was the perfect excuse for Darius to invade the Greek mainland. As cities such as Athens aided the rebel cities, Darius was able to construe this as an act of war and wage his campaign to expand his empire. Unfortunately for Darius, he did not count on the Greeks' resolve and knowledge of their own terrain, which resulted in his defeat at the Battle of Marathon. 

The Warring States Period of Chinese history lasted from approximately 476 BCE until 221 BCE It was a period of disorder and civil conflict in China as various states rose and declined in a seemingly interminable struggle for power. The 250 years of conflict came to an end in 221 BCE with the conquests of the Qin and the establishment of the short-lived Qin Dynasty under Qin Shi Huangdi.

While all of these tribes contributed in their own way to the breakdown of Roman power, they did so either fleeing or allying with the incoming Huns from Asia, who rode in on horseback, conquered most of Europe, and made Rome pledge allegiance to them. After the death of Attila the Hun, disease and infighting weakened the Hun's control of the Western Empire enough for their former vassals to break their control in 454 CE.

Their lasting influence was an enormous shakeup of European powers, tribal territories, and the introduction of horses and related technologies and techniques.

Following the suicide of the Emperor Nero, the Roman Empire was thrown into flux. For the first time since Augustus, there was no clear successor to the throne and civil war ensued. Throughout 69 CE, Galba, Otho, Vitellius and Vespasian all holding power in areas of the empire. Vespasian would eventually win and start a dynasty of his own. 

The Greeks collided the most with the Persians. These conflicts began when Cyrus the Great of Persia conquered a portion of Greece in 547 BCE. This incited a series of wars, which the Greeks ended up coming out victorious. The Greeks did not experience conflict with the Mauryans or the Qin dynasty because they were geographically separated by great distance. Furthermore, the Greeks and Romans actually engaged in a great deal of cross-cultural interactions.

The Persian Wars began in 499 BCE and lasted until 450 BCE. The conflict was fought between the Persian Empire and various Greek city-states and culminated in victory for the Greeks at the Battle of Marathon. The wars began when Ionian Greeks, living under Persian control, in modern-day Turkey, revolted against the Persian authorities. The Greek city-states offered their support and earned the ire of the Persian emperors who vowed to conquer all of Greece.

The Peloponnesian War was fought between the forces of Athens and Sparta from 431 BCE until 404 BCE The war began as the culmination of centuries of rivalry between Athens, Sparta, and the other Greek city-states for dominance in the Aegean. It ended in the annihilation of the Athenian army and fleet and the destruction of Athens.

Julius Caesar crossed the Rubicon in 49 BCE, specifically disobeying the Senate and practically declaring war on the Republic. Pompey was one of Julius's allies, although he himself did not cross the river along with Caesar. Augustus and Antony were allies, and then subsequently enemies, after the fall of Julius, however they too did not participate in the historic crossing. Ovid famously authored the Metamorphoses, but he was not a general, so he would not be a good choice.

The Battle of Marathon took place in 490 BCE, part of the initial war between the Greek city-states and the Persian Empire. The battle ended in an overwhelming defeat of the invading Persians by a combined force of Greek city-states led by Athens. It was a turning point in the war, as the Greeks managed to gain the upper hand for the first time since conflict had begun.