Indus Valley Civilization (c. 3300–1300 BC, flowered 2600–1900 BC), abbreviated IVC, was an ancient riverine civilization that flourished in the Indus river valley in Pakistan. Another name for this civilization is the Harappan Civilization, after the first excavated city of Harappa.

Ruins of Harappa and MohenjoDaro (Pakistan)

The ruins of Harappa were first described by Charles Masson in his
Narrative of Various Journeys in Balochistan, Afghanistan and Punjab,
1826-1838; however, its significance was not realized until much
later. Moreover, in 1857, British engineers unwittingly employed bricks
from the Harappa ruins in the construction of the East (British) Indian
Railway line connecting Karachi and Lahore. More than half a century
later, in 1912, Harappan seals—with the then unknown symbols—were
discovered by J. Fleet, prompting an excavation campaign under Sir John
Hubert Marshall in 1921/22, and resulting in the discovery of the
hitherto unknown civilization by Dayaram Sahni. By 1931, much of
Mohenjo-Daro had been excavated, but minor campaigns continued, such as
that led by Mortimer Wheeler in 1950.

The mature phase of the Harappan civilization lasted from c. 2600 BCE
to 1900 BCE. With the inclusion of the predecessor and successor
cultures—Early Harappan and Late Harappan, respectively—the entire Indus
Valley Civilization may be taken to have lasted from the 33rd to the
14th centuries BCE. Two terms are employed for the periodization
of the IVC: Phases and Eras. The Early Harappan, Mature
Harappan, and Late Harappan phases are also called the
"Regionalisation," "Integration," and "Localisation" eras,
respectively, with the Regionalization era reaching back to the
Neolithic Mehrgarh II period:

Early Harappan

The development of these farming communities ultimately led to the
accretion of larger settlements from the later 4th millennium.

The Early Harappan Ravi Phase, named after the nearby Ravi River,
lasted from circa BC 3300 until BC 2800. Some of the most important
discoveries in the Ravi Phase relate to writing. The earliest examples
of the Indus script date from around BC 3000, placing the origins of
writing in South Asia at approximately the same time as those of Ancient
Egypt and Mesopotamia.

The mature phase of earlier village cultures is represented by Rehman
Dheri and Amri. Kot Diji (Harappan 2) represents the phase leading up to
Mature Harappan, with the citadel representing centralised authority and
an increasingly urban quality of life.

This distinctive, regional culture which emerged is called Early or
Pre-Harappan. Trade networks linked this culture with related regional
cultures and distant sources of raw materials, including lapis lazuli
and other materials for bead-making. Villagers had, by this time,
domesticated numerous crops, including peas, sesame seeds, dates and
cotton, as well as a wide range of animals, including the water buffalo.

Mature Harappan

By BC 2500, the Early Harappan communities had been turned into urban
centers. Such urban centers include Harappa and Mohenjo Daro in Pakistan
. In total, over 1,052 cities and settlements have been found, mainly in
the general region of the Indus Rivers and their tributaries. By 2500
BCE, irrigation had transformed the region.

Late Harappan

Around 1800 BCE, signs of a gradual decline began to emerge, and by
around 1700 BCE, most of the cities were abandoned. However, the Indus
Valley Civilisation did not disappear suddenly, and many elements of the
Indus Civilization can be found in later cultures. Current
archaeological data suggests that material culture classified as Late
Harappan may have persisted until at least c. 1000-900 BCE, and was
partially contemporaneous with the Painted Grey Ware and perhaps early
NBP cultures. Archaeologists have emphasised that there was a continuous
series of cultural developments that link "the so-called two major
phases of urbanisation in South Asia".

A possible natural reason for the IVC’s decline is connected with
climate change: The Indus valley climate grew significantly cooler and
drier from about 1800 BCE. A crucial factor may have been the
disappearance of substantial portions of the Ghaggar Hakra river system.
A tectonic event may have diverted the system’s sources toward the
Ganges Plain, though there is some uncertainty about the date of this
event. Although this particular factor is speculative, and not generally
accepted, the decline of the IVC, as with any other civilisation, will
have been due to a combination of various reasons.

The region lies on the ancient route used by successive waves of
migrations from Aryans to Huns, and later by Turks and Mughals to South
Asia over the passes in the Hindu Kush. The Swat culture of northern
Pakistan is a likely candidate for the first settlements of Indo-Aryans
in the subcontinent. It is in this context of the aftermath of a
civilisation’s collapse that the hypothesis of an Indo-Aryan migration
into northern India is discussed. In the early twentieth century, this
migration was forwarded in the guise of an "Aryan invasion", and when
the civilization was discovered in the 1920s, its collapse at precisely
the time of the conjectured invasion was seen as an independent
confirmation. In the words of the archaeologist Mortimer Wheeler, the
Indo-Aryan war god Indra "stands accused" of the destruction.