The campaign of Alexander the Great in the East and the collapse of the Achaemenid Persian empire marked the beginning of series of upsets in Central Asia and India. Firstly, the unexpected death of Alexander in 323 BC let his multicultural empire weak, soon the prey of his former companions who bothered more for Near-Eastern regions than for what was the Greek Far-East. Secondly, the relationships between northern nomads and settled kingdoms South that existed during Achaemenid period were broken. This, alongside the weakening of Successors states, let an increasing number of nomads incursions takes place in the area, even to India. Thirdly, the first great empire of India, the Mauryan one, was created shortly after the death of Alexander, in 321 BC. But, despite this period of strength, North-Eastern India became divided again after Ašoka’s death in 232 BC, making it a breeding ground for northern invaders.
The result was that the Seleucid kingdom, the one who succeeded to finally take the Eastern province of former Alexander’s realm, wasn’t able to keep it for long. Despite several campaigns of Seleucos and Antiochos against nomadic intrusions, Seleucid presence was too weak to prevent two satraps from revolting, Andragoras in Parthia and Diodotos in Bactria, both near 250 BC. The first one was quickly dethroned by some northern nomads, the Parni, who took the name of their new subjects, the Parthians. Diodotos had more success, creating a kingdom which expanded quickly, withstanding Seleucid and nomads invasions to even gain some territories in North-East India. Uprisings and strifes seem nevertheless to have been the sins of this empire, soon divided between rival families. We commonly use, for lack of something better, the term Greco-Bactrians for the kings reigning North and West of the Hindu-Kush, and Indo-Greeks for those in the Indian subcontinent. The first ones suffered defeats at the hand of Parthians and nomads to be finally vanquished in the early 120s BC. Indo-Greek kingdoms lasted longer, gradually shortened by other neighbouring kingdoms.
In the meantime, northern nomads pushed themselves in a “domino effect”, the Yuezhi from beyond the Tarim bearing down the Sakas of the steppes. The first ones finally settled in Bactria, as the Sakas invaded today Afghanistan and Pakistan to form several kingdoms, those in India being commonly called “Indo-Saka kingdoms”. Some merged with mysterious Parthian rulers to create the “Indo-Parthian kingdoms”. This pretty complex situation only ended in the first century AD, when the Yuezhi, finally merged under the Kushan dynasty, invaded and anihilated almost all of the Indo-Saka and Indo-Parthian kingdoms to create a large and powerful empire, able to set itself at the same level than Roman, Parthian and Chinese Han ones. This empire extended from Sogdiana to Central India, lasting until the Sassanian uprising in Iran that also crushed Kushan forces.
Central Asian history in Hellenistic and Imperial times is complex and difficult to understand. This is due to the geopolitical instability of the period, but also to the lack of sources for the whole period of time. Excavations are difficult to make due to political situation today, especially in Afghanistan and Pakistan, so discovers are made slowly, and often on the black market. That’s the reason why we can have several dates for the same king and why frontlines and succession orders are so divergent among scholars. It is something to keep in mind when reading books and articles on the period.