Arachosia's boundaries varied with successive rulers, but it may have once corresponded to much of present-day southeastern Afghanistan and northern Pakistan, perhaps even extending all the way eastwards to the Indus River (see geography for details). Its center lay in what is today the Arghandab District of Afghanistan.
"Arachosia" was named after the name of a river that runs through it, the Arachotus (ˈærəˌkoʊt; Αραχωτός, Arakhōtos), today known as the Arghandab, a tributary of the Helmand.
In his list, Ptolemy also refers to a city named Arachotus (ˈærəˌkoʊt; {{lang-el|Ἀραχωτός}) or Arachoti (acc. to Strabo), which was the earlier capital of the land. Pliny the Elder and Stephen of Byzantium mention that its original name was Cophen (Κωφήν). This city is identified today with Arghandab that lies just north of present-day Kandahar.
Although centered around the Arghandab Valley, the extent of Arachosia remains unclear. According to Ptolemy (6.20.1, cf. Strabo 15.2.9), Arachosia was bound by Drangiana in the west, the Paropamisadae (Gandara) in the north, to "a part of India" in the east, and Gedrosia in the south. Strabo (11.10.1) suggests that Arachosia extended eastwards as far as the Indus river. Pliny (Natural History 6.92) speaks of Dexendrusi in the south.
The chronologically next reference to Arachosia comes from the Greeks and Romans, who record that under Darius III the Arachosians and Drangians were under the command of a governor who, together with the army of the Bactrian governor, contrived a plot of the Arachosians against Alexander (Curtius Rufus 8.13.3). Following Alexander's conquest of the Achaemenids, the Macedonian appointed his generals as governors (Arrian 3.28.1, 5.6.2; Curtius Rufus 7.3.5; Plutarch, Eumenes 19.3; Polyaenus 4.6.15; Diodorus 18.3.3; Orosius 3.23.1 3; Justin 13.4.22).
Following the Partition of Babylon, the region became part of the Seleucid Empire, which traded it to the Mauryan Empire in 305 BCE as part of an alliance. The Sunga Dynasty overthrew the Mauryans in 185 BC, but shortly afterwards lost Arachosia to the Greco-Bactrian Kingdom. It then became part of the break-away Indo-Greek Kingdom in the mid 2nd century BCE. Indo-Scythians expelled the Indo-Greeks by the mid 1st century BCE, but lost the region to the Arsacids and Indo-Parthians. At what time (and in what form) Parthian rule over Arachosia was reestablished cannot be determined with any authenticity. From Isidore 19 it is certin that a part (perhaps only a little) of the region was under Arsacid rule in the 1st century CE, and that the Parthians called it Indikē Leukē, "White India."
The Kushans captured Arachosia from the Indo-Parthians and ruled the region until around 230 CE, when the they were defeated by the Sassanids, the second Persian Empire, after which the Kushans were replaced by Sassanid vassals known as the Kushanshas or Indo-Sassanids. In 420 CE the Kushanshas were driven out of Afghanistan by the Chionites, who established the Kidarite Kingdom. The Kidarites were replaced in the 460s CE by the Hephthalites, who were defeated in 565 CE by a coalition of Persian and Turkish armies. Arachosia became part of the surviving Kushano-Hephthalite Kingdoms of Kapisa, then Kabul, before coming under attack from the Moslem Arabs. Around 870 CE the Kushano-Hephthalites (aka Turkshahi Dynasty) was replaced by the Hindu-shahi dynasty, which fell to the Muslim Turkish Ghaznavids in the early 11th century CE.
Arab geographers referred to the region (or parts of it) as 'Arokhaj', 'Rokhaj', 'Rohkaj' or simply 'Roh'.
The south, southeast and northeast parts of Arachosia retained Buddhist-Hindu religious and cultural influence until the advent of Islam in the 7th century. Much of the country remained Buddhist even while in Arab hands, but within a few centuries Islam became the region's dominant religion. See Sistan for information on the religion of the area after the Arab conquest.