The Medes were allies of the Neo-Babylonian Empire, and in 612 BCE they took part in the huge rebellion that resulted in the downfall of the Assyrian Empire. For just over fifty years, the Medes continued to dominate the Iranian plateau. Then, in 550 BCE a Persian leader, Cyrus, led the Persians against the Medes and conquered them (practically speaking, there was little distinction between the two groups since they were so closely-related and similar; the Greeks regularly confused the two when writing about them). He assimilated the Medes into his own military force and then embarked on an incredible campaign of conquest that lasted twenty years, forging Persia into a gigantic empire.
Cyrus began his conquests by invading Anatolia in 546 BCE, conquering the kingdom of Lydia in the process. His principal further west were the Greek colonies of Ionia, along the coast of the Aegean Sea. Cyrus swiftly defeated the Greek poleis, but instead of punishing the Greeks for opposing him he allowed them to keep their language, religion, and culture, simply insisting they give him loyal warriors and pay taxes. He found Greek leaders willing to work with the Persians and he appointed them as governors of the colonies. Thus, even though they had been beaten, most of the Greeks in the colonies did not experience Persian rule as particularly oppressive.
Cyrus next turned south and conquered the city-states and kingdoms of Mesopotamia, culminating with his conquest of Babylon in 539 BCE. This conquest was surprisingly peaceful; Babylon was torn between the priests of Marduk (the patron deity of the city) and the king, who was trying to favor the worship of a different goddess. After he defeated the forces of the king in one battle, Cyrus was welcomed as a liberator by the Babylonians and he made a point of venerating Marduk to help ensure their ongoing loyalty.
Much of what historians know about Persia is gleaned from the propaganda Persian kings left behind. The conquest of Babylon produced an outstanding example - the “Cyrus Cylinder,” a pillar covered in a proclamation that Cyrus commissioned after the conquest of Babylon.
Cyrus continued the practice of finding loyal leaders and treating his conquered enemies fairly, which kept uprisings against him to a minimum. He then pushed into Central Asia, in present-day Afghanistan, conquering all of what constituted the “known world” in that region. To the northeast were the steppes, home of a steppe nomad people called the Scythians, whom the Persians would go on to fight for centuries (Cyrus himself died in battle against the Scythians in 530 BCE - he was 70 years old at the time).
Cyrus was followed by his son Cambyses II. Cambyses led the Persian armies west, conquering both the rich Phoenician cities of the eastern Mediterranean coast and Egypt. He was installed as pharaoh in Egypt, again demonstrating Persian respect for local traditions. Thus, in less than thirty years, Persia had gone from an obscure kingdom in the middle of the Iranian plateau to the largest land empire in the entire world, bigger even than China (under the Eastern Zhou dynasty) at the time. Cambyses died not long after, in 522 BCE, under somewhat mysterious circumstances – he supposedly fell on his sword while getting off of his horse.
In 522, following Cambyses’ death, Darius I became king (r. 521 – 486 BCE). Darius came to power after leading a conspiracy that assassinated Cambyses’ younger brother Bardiya, who had briefly ruled. By the time Darius seized the throne, the Persian Empire was already too large to rule effectively; it was bigger than any empire in the world to date but there was no infrastructure or government sufficient to rule it consistently. Darius worked to change that. He expanded the empire further and, more importantly, consolidated royal power. He improved infrastructure, established a postal service, and standardized weights, measures, and coinage. He set up a uniform bureaucracy and system of rule over the entire empire to standardize taxation and make it clear what was expected of the subject areas.
Darius inherited the conquests of his predecessors, and he personally oversaw the conquest of the northern part of the Indus river valley in northwestern India, thus marking the first time in world history when one state ruled over three of the major river systems of ancient history (i.e. the Nile, Mesopotamia, and the Indus). In 513 BCE he oversaw a gigantic invasion of Central Asia to try to end the raids of the Scythians once and for all; he was forced to retreat without winning a decisive victory, but his army was still intact and he had added Thrace (present-day Bulgaria) to the empire.
Darius was also interested in seizing more territory to the west, conquering the remaining Greek colonies on the coast of Anatolia. In 499 BCE several Ionian Greek poleis rose against the Persians and successfully secured Athenian aid. Several years of fighting followed, with the Persians eventually crushing the rebellion in 494 BCE (the Persians deported many of the Greek rebels to India as punishment). Athens’ decision to support the rebellion angered the Persians, however, and Darius began to plan a full-fledged invasion of Greece.