The Persians and the Medes were distantly related to the Scythians, the Hittites, the Greeks and the Romans, and they spoke a related language. Like the Scythians, the Medes and the Persians were nomadic people. They travelled around Central Asia with their horses and their cattle, and grazed the cattle and the horses on the great fields of grass there. Usually they lived well enough this way.
But sometimes the weather was worse than usual, and the Medes and Persians could not find enough to eat. This time, when that happened, the Medes and Persians headed south into West Asia. Maybe they had heard that there were Dark Ages there and they thought it would be easy to take over. Maybe they just thought it would be nicer in the south, where it was warmer.
The Medes and the Persians settled in what is now Iran, and we don't hear much about them until about 600 BC. Probably they could not fight the Assyrians and didn't try to. But by 600 BC the Assyrians were getting weaker. At this time the Medes and the Persians combined into one group, under one king.
Tomb of Cyrus the Great
At first the Medes were in charge, but in 559 BC Cyrus, who was a Persian, made himself king, and from then on the Persians were in charge. Cyrus (SIGH-russ) soon also conquered the whole rest of West Asia: the Babylonians, the Assyrians, the Jews, the Phoenicians and the Syrians, and also the Lydians and the Greeks in modern Turkey. He is remembered as a good ruler. He managed to unify a very diverse group of people, with many different languages and religions. At the same time, he allowed each group to keep their own religion. This is especially surprising because he himself had recently converted to Zoroastrianism and clearly felt strongly about his new faith.
When Cyrus died in 530 BC, his son Cambyses (cam-BYE-sees) became king. Cambyses added Egypt to the Persian Empire, beating an Egyptian army that also had many Greek soldiers fighting for pay. But according to Herodotus Cambyses suffered from severe mental illness later in his life, and eventually his own people killed him.
In 521 BC Darius (da-RYE-us), who was a Persian and a Zoroastrian but only a distant cousin of Cyrus and Cambyses, seized the throne. He moved the Persian capital to the new city of Persepolis, and hired workmen from all over to work on the new buildings there. Sculptors came from as far away as Greece to work on the Persian palace at Persepolis.
Darius also tried to conquer the Scythians, but failed.
In 490 BC, Darius tried to conquer Athens and mainland Greece. Some of the Greek cities, like Thebes, surrendered to Darius or made treaties with him. But Athens fought back and defeated the Persians, and Darius took his troops and went home.
The next Persian king, Xerxes (ZERK-sees), put down a big rebellion in Egypt and then attacked Greece again in 480 BC. But Xerxes was also defeated, and went home. The Persians pretty much stopped trying to expand their empire then. But they continued to rule from Afghanistan to Turkey and Egypt for another 150 years, until they were conquered by Alexander the Great.