Almost everything most people know about Punjab happens in its last five hundred years. The Gurus, the Khalsa, the empire, the border, the aeroplanes out — all of it is recent. Underneath sits a much longer stretch in which the land was already one of the most worked-over places on earth, and it is worth knowing about, because it explains why everybody who came afterwards wanted it.
The rivers came first
The organising fact of this region is water. Five rivers — the Jhelum, Chenab, Ravi, Beas and Sutlej — come down off the Himalaya and run southwest into the Indus, and the flat, silt-rich country between them is the reason people settled here at all. The name itself is late and Persian, panj-āb, five waters. The Rigveda, composed in this landscape, calls it something older: Sapta Sindhu, the seven rivers.
Around 2600 BCE the cities of the Indus civilisation were built on these plains, with drains, standardised bricks and a script nobody has read. One of the two great ones, Harappa, stands in what is now Sahiwal district in Pakistani Punjab. Much of it went into railway ballast in the nineteenth century before anyone understood what it was.
The Greeks, and the grammarian
In 326 BCE Alexander crossed the Jhelum and fought Porus somewhere on its far bank — a battle in the rain, against war elephants, that he won and then handed the kingdom back. His army got as far as the Beas and refused to go further. That is where the Greek world stopped, and it stopped in Punjab.
What followed was more interesting than the invasion. Greek dynasties settled into the northwest for two centuries; one of their capitals was Sagala, generally identified with Sialkot, where the king Menander appears in a Buddhist text arguing philosophy with a monk. Up in the Potohar, Taxila was a centre of learning drawing students from across the subcontinent, and somewhere near Attock, probably in the fourth century BCE, a man named Panini wrote a description of Sanskrit so exact that linguists still work through it.
Multan was already ancient by then — a city with a famous sun temple that pilgrims and raiders both came for. Jalandhar, in the country between the Beas and the Sutlej, carries the name of an old kingdom, Trigartta, that was here long before the Doaba was a place people emigrated from.
Then everybody, in turn
After the Greeks: Sakas, Parthians, Kushans, Guptas, Huns. Punjab is a plain with passes at one end of it, which means it is a corridor, which means it gets walked through. Nothing in this era stays settled for two hundred years running. The pattern that holds for the next thousand years — an invasion from the northwest, a battle on one of the five rivers, a new dynasty in Lahore or Multan — is already fully established well before the year 1000.
What comes next
Around 1000 CE the direction of arrival changes and the arrivals stop leaving. Mahmud of Ghazni takes Lahore, the Sufis begin coming down the same roads as the armies, and the next era — Sultanate & Mughal — starts with a city becoming a capital and ends with ten Gurus having remade what it meant to be Punjabi.
This is the thinnest era on the site, and deliberately the next one we are writing: Harappa, the Hydaspes, Taxila, Panini and the Indo-Greeks all have articles coming.