Around 300 BC, some new people
invaded West Asia from Siberia in the north. These people were called the
Parthians. Like the Scythians, and like the
Persians when they first came to West Asia, the
Parthians were nomadic people.
They travelled around Siberia 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 Parthian cattle could not find enough to eat. This time, when that happened, the Parthians headed south into Alexander's empire. Maybe they had heard that Alexander had died 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 Parthians immediately succeeded in taking over the middle part of Alexander's empire (roughly modern Iran). This split the Greek empire in half, leaving the Greek colonies in Bactria (modern Afghanistan) isolated. They stayed there for about 200 years, gradually learning the culture of West Asia. They converted to Zoroastrianism.
Parthian temple of Mithra, at Hatra (Iraq) 200 BC
But around 100 BC, Seleucia was getting weaker and weaker. The Parthians started to take over parts of Eastern Seleucia. At the same time, the Romans started to take over parts of Western Seleucia. Eventually the Romans and the Parthians met in the middle. There was a great battle which the Parthians won (by treachery, the Romans said), and Parthian soldiers killed the Roman general, Crassus. Possibly the Parthians' advantage was that they had the new Indian crucible steel swords.
From then on, for hundreds of years, the Romans and the Parthians were always more or less fighting about where exactly the border between them would be. Sometimes the Romans won, sometimes the Parthians won.
Ancient Persia, by Don Nardo (2003). Good for reports.
Parthian Art, by Malcolm Colledge (1977). Not very easy to get anymore, but it's the classic book for the Parthians - it has a lot more than just the art.
Ancient Persia, by Josef Wiesehofer (2001). Includes the Parthians and the Sassanians.
The Cambridge History of Iran: Volume 3, The Seleucid, Parthian and Sasanid Periods, Part 2: Seleucid Parthian, edited by E. Yarshater (reprinted 1983). Expensive but complete.