The first superpower and its founder
In 549 BC the city of Ecbatana, capital of the Median Empire, fell to the Persians and the Median king Astyages was deposed. The Median Empire had been one of the four big powers of the Middle East, the others being Babylonia, Lydia, and Egypt. The Persians had been a minor people, divided between the kingdoms of Persia and Anshan -- in fact, they had been vassals of the Median Empire, so the Persian war on Media was really a rebellion against an overlord. After this startling victory the Persian king, Cyrus, took Astyages's place as ruler of the Medes.
(I'm citing all proper names in their Greco-Latin forms because those are what you'll see in most English-language sources on this period. The forms in the original Middle Eastern languages were quite different.)
One detail of what followed was almost equally surprising. The normal treatment meted out to overthrown kings at that time was death by impalement, but Cyrus merely packed Astyages off into retirement with a pension.
The Medes were a people closely related to the Persians and probably saw the accession of Cyrus as more a mere change of leadership than a foreign conquest -- the distinction between the two counted for so little that later the Greeks indiscriminately referred to Persians as "the Medes", and modern Persian-speaking Iranians are probably descended from both. Cyrus went on to an impressive series of further conquests of quite different states and peoples including Lydia, the great Babylonian Empire including what we now call Syria and Israel (at that time Babylon was the world's largest city and most advanced civilization), and the wilder tribal peoples to the east and northeast of Iran proper. In just 20 years he built an empire comparable in size to that of the much later Romans. No state even approaching such a size had ever existed before (even China, ancient as a culture, did not become a unified state until 221 BC). He did not conquer Egypt, but his son and successor Cambyses added it to the Persian Empire a few years after his death. He is known to history as "Cyrus the Great".
The other strategy was exploitation of local religions. In Babylon, for example, Cyrus capitalized on suspicions that the existing king Nabonidus was not a legitimate heir to the throne by declaring that the Babylonian god Marduk had sought him out to liberate the "noble Babylonian race" from Nabonidus's oppression, and to restore the temples which the old king had neglected. The Persian Empire rapidly developed a sophisticated propaganda department which found ways to use the religion of each conquered country to legitimize Persian rule.
An example of Cyrus's style of rule is found in the Old Testament. After the conquest of Babylon, it was he who freed the Israelites from captivity there to return to their own land, and even paid for the rebuilding of the Jerusalem temple. Since Egypt was still an independent and rival state in his lifetime, it was useful to him to have a loyal population in Israel near his border with Egypt.
Cyrus was the true inventor of the giant multicultural universal state, a form identified in the Western mind with the Romans. The Persian Empire ultimately unified an astonishing variety of peoples -- Central Asian nomads, Ionian Greeks, seafaring Phoenecians, Arab Bedouins, monotheistic Hebrews, Indus Valley kingdoms, the (even then) ancient civilization of Egypt, the business-savvy metropolis of Babylon, and many others. It extended from the Indus river in modern Pakistan to Cyrenaica in modern Libya, an east-west span greater than that of the modern continental US.
In modern times we think of empires as oppressive and instinctively side with smaller groups opposing the rule of more powerful ones, but in ancient times, when the independence of small states usually meant constant pointless warfare, incorporation into a giant state with the power to keep peace and order meant much better lives for most people, however humiliating it may have been to their local rulers.
The pax Persica helped spread ideas and knowledge. Babylonian banks (a concept unknown elsewhere) set up branches throughout the Empire, something which would have been impossible under a hodgepodge of squabbling local states. As travel became safer and easier, Greek scholars in increasing numbers went to Babylon to study its astronomy, medicine, and history, helping to trigger the spectacular rise of Greek science. A vast system of qanats (underground aqueducts) irrigated the arid Iranian plateau (thousands of them are still in use today, 25 centuries later); under Persian rule, the qanat system was introduced into Egypt.
Cyrus the Great was not by any stretch of the imagination a modern democrat or liberal. He was a product of his time; he ruled as a king and demanded the respect due to one. But he was a startlingly just and humane man for his time, and he left the Middle East a much better place than he found it.
The Empire faced its first major crisis a few years after Cyrus's death, when a minor member of the royal family took the throne as Darius I after the deaths of Cyrus's two sons (the history of these events is murky and intriguing, but they were both probably killed by Darius). Revolts broke out against the perceived usurper, but Darius managed to hold the Empire together and went on to become its most capable and important king after Cyrus himself.
But that conflict was ultimately the Empire's doom. When Alexander the Great invaded it in 334 BC, his goal was not to destroy it but to capture it intact. Alexander, one of the greatest generals who ever lived, took Cyrus the Great as his role model and dreamed of establishing a common civilization of Greeks, Persians, and the other peoples of the Empire and someday beyond. What the world could have gained if he had succeeded, we will never know. But he died young, with no clear successor; his generals divided the Empire, and the Middle East fell back into an age of quarreling smaller states.
(For another episode from Persian history, a thousand years after Cyrus but still long before the coming of Islam, see here.)