I know there's no consensus and likely never will be on all the causes for the various invasions of Eurasian nomads (Alans, Huns, Mongols et al.) but it seems to be a combination of climate change and being pushed West by the conquests of Imperial China.
Are there then any known examples of tribes being pushed East by the conquests of say Greece or Rome?
I imagine this almost surely occurred during Alexander's campaign, are there records from India of seemingly random tribal invasions immediately preceding the arrival of the Greek army?
Just to be clear: I realize there's a strong argument that Alexander was the nomadic invader.
In 21st century BC, South Mesopotamia fell to the semi-nomadic semitic people known as Amorites, who came from northern Levant (Syria), around thousand kilometers north-west from S-Mesopotamia.
Amorites created their own dynasties there, but the reason how they got so powerful at all was due to the power vacuum in Mesopotamia, which was in chaos ever since the fall of the Akkadian Empire in 22th century BC. The lands between Euphrates and Tigris were still rich and fertile, and with highly civilized city states with amazing culture and technology of its time, but also militarily weak because of all that political turmoil. So in this case, the nomads were pulled away from their mountainous homeland to Mesopotamia due to more suitable external conditions.. or rather driven from west to east by desires for a prosperous and wealthy land.
The period in Mesopotamia after the fall of the Akkadians is called the Amorite period or the era of Amorite kingdoms. (Lasted up to 17th century BC) The kingdoms were the city-states, which they ruled, and one of those city-states became so powerful, that it in time defeated all others and established an empire great enough to have its fame endure the test of time.
Wonder what king Hammurabi would have thought, if he had known that he, his infamous "Code of law" and his legendary Babylonian State are still taught to the children of the future in schools 39 centuries after his death, in 21st century CE.