The name Cyrus is a Latin transliteration of the Greek . The ancient historians Ctesias and Plutarch noted that Cyrus was named from Kuros, the sun, a concept which has been interpreted as meaning "like the sun," by noting its relation to the Persian noun for sun, khorsheed, while using -vash as a suffix of likeness. However, some modern historians, such as Karl Hoffmann and Rüdiger Schmitt of the Encyclopædia Iranica, have suggested the translation "humiliator of the enemy in verbal contest."

In modern Persian, Cyrus is referred to as Kourosh-e Bozorg — the Persian-derived name for Cyrus the Great. In the Bible, he is known as simply Koresh.

The exact dates of the Lydian conquest are unknown, but it is generally suggested to have begun in 547 BC. The Lydians first attacked the Achaemenid Empire's city of Pteria in Cappadocia. Croesus laid siege to the city, and captured its inhabitants as slaves. Meanwhile, The Persians invited the citizens of Ionia, who were part of the Lydian kingdom, to revolt against their ruler. The offer was rebuffed, and thus Cyrus levied an army and marched against the Lydians, increasing his numbers while passing through nations in his way. The Battle of Pteria was effectively a stalemate, with both sides suffering heavy casualties by nightfall. Croesus retreated to Sardis the following morning.

While in Sardis, Croesus sent out requests for his allies to send aid to Lydia. However, near the end of winter, before the allies could unite, Cyrus pushed the war into Lydian territory and besieged Croesus in his capital, Sardis. Shortly before the final battle between the two rulers, Harpagus advised Cyrus to place his dromedaries in front of his warriors; the Lydian horses, not used to the dromedaries' smell, would be very afraid. The strategy worked; the Lydian cavalry was routed. Cyrus defeated and captured Croesus. Cyrus occupied the capital at Sardis, conquering the Lydian kingdom in 546 BC. According to Herodotus, Cyrus spared Croesus' life and kept him as an advisor, but this account conflicts with some translations of the contemporary Nabonidus Chronicle, which interpret that the king of Lydia was slain.

Before returning to the capital, a Lydian named Pactyes was entrusted by Cyrus to send Croesus' treasury to Persia. However, soon after Cyrus' departure, Pactyes hired mercenaries and caused an uprising in Sardis, revolting against the Persian satrap of Lydia, Tabalus. With recommendations from Croesus that he should turn the minds of the Lydian people to luxury, Cyrus sent Mazares, one of his commanders, to subdue the insurrection, but demanded that Pactyas be returned alive. Upon Mazares' arrival, Pactyas fled to Ionia, where he had hired mercenaries. Mazares marched his troops into the Greek country and captured the cities of Magnesia and Priene, where Pactyas was captured and sent back to Persia for punishment.

Mazares continued the conquest of Asia Minor, but died of unknown causes during his campaign in Ionia. Cyrus sent Harpagus to complete Mazares' conquest of Asia Minor. Harpagus captured Lycia, Cilicia and Phoenicia, using the technique of building earthworks to breach the walls of besieged cities, a method unknown to the Greeks. He ended his conquest of the area in 542 BC, and returned to Persia.

In 539 BC, towards the end of September, Cyrus' armies, under the command of Gubaru, the governor of Gutium, attacked Opis on the Tigris river and defeated the Babylonians after a minor uprising. With Opis subjugated, the Persians took control of the vast canal system of Babylonia.

On October 10, the city of Sippar was seized without a battle, with little to no resistance from the populace. It is probable that Cyrus engaged in negotiations with the Babylonian generals to obtain a compromise on their part and therefore avoid an armed confrontation. Nabonidus was staying in the city at the time, and soon fled to the capital, Babylon, which he had not visited in years.

Two days later, on October 12, Gubaru's troops entered Babylon, again without any resistance from the Babylonian armies. Herodotus explains that to accomplish this feat, the Persians diverted the Euphrates river into a canal so that the water level dropped "to the height of the middle of a man's thigh," which allowed the invading forces to march directly through the river bed to enter at night. On October 29, Cyrus himself entered the city of Babylon and arrested Nabonidus. He then assumed the titles of "king of Babylon, king of Sumer and Akkad, king of the four sides of the world."

Prior to Cyrus' invasion of Babylon, the Babylonian Empire had conquered many kingdoms. In addition to Babylonia itself, Cyrus incorporated its subnational entities into his Empire, including Syria and Palestine.

Before leaving Babylon, Cyrus also freed the Israelites by allowing them to return to their native land, effectively ending the Babylonian captivity. The return of the exiles reinforced the Jewish population in their homeland, which had been waning since the start of the Babylonian rule.

According to the Behistun Inscription of Darius the Great, Cyrus' dominions must have comprised the largest empire the world had ever seen. At the end of Cyrus' rule, the Achaemenid Empire stretched from Asia Minor and Judah in the west to the Indus River in the east.

Upon taking Babylon, Cyrus issued a declaration inscribed on a clay barrel, known today as the Cyrus Cylinder. It recounts his victories and merciful acts, and documents his royal lineage. It was discovered in 1879 in Babylon, and today is kept in the British Museum.

While the cylinder reflects a long tradition in Mesopotamia where, as early as the third millennium BC, kings such as Urukagina began their reigns with declarations of reforms, the Cyrus Cylinder has been referred to as the "first charter of human rights"; though such a concept would have been alien to Cyrus and his contemporaries. The cylinder decrees the normal themes of Persian rule: respect for the gods of all peoples, just and peaceful rule, and the power and glory of the empire.


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