The conquest of the territories of the Indian sub-continent to west of the Hindus by Darius I, according to the Bisotun inscription, can be dated back to around 518 B.C. (Vogelsang 1987: 187-188; Briant 1996: 153); a slow penetration in different steps, starting from north to south (Fussman 1993: 84) had been proposed and confirmed by the fact that the toponym of Hindus appears only in the later inscriptions, whilst the Gandharians are already mentioned in Bisotun. As regards as to the chronological extent of the Achaemenid domination, at least to north-west of the Indian sub-continent, different opinion exist amongst scholars: that the region was again independent (Chattopadhyaya 1974: 25-26), already from the end of the Darius I’s kingdom or during that of Artaxerses II, does not seem to be confirmed by Ctesias, who mentioned gifts of the Kings of India, and however also Darius III had, amongst his army, Indians contingents (Briant 1996: 699, 774). That officers of the Great King were not explicitly nominated at the moment of the arrival in the area of the Macedonian army (327-26 B.C.), cannot be interpreted as an absence of the Achaemenid power, as still recently has been proposed (Dittmann 1984: 185); a large amount of other set of data give indication, instead, that in different forms from those of the epoch of Darius I, the Persian power still had a control there (Briant 1996: 776-778).
According to Ctesia, doctor in the Artaxerxes II’s court, the kings of India sent to the Persian King precious essences (Henry 1947: 84), animals and valuable cloths (Aelian, N.A., 4, 21 e 46). An interesting evidence of the contacts between Achaemenid Iran and India is also constituted by some tablets found in Persepolis, reminding the names of Gandharians, Indians and Arachosians present at Persepolis or of officers of the King sent to East, to whom food portions had been given (Vogelsang 1990: 101; Seibert 2002: 22).
The peoples inhabiting the historical regions of this “Indo-Iranian frontier” are known already in the list of the Achaemenid satrapies, as given by Herodotus (3, 89-94) and, before him, by Hecateus of Miletus. This one certainly knew the Indian population of Gandharai and the city of Kaspapyros (FGrHist, I, frg. 294-295). The easternmost satrapies were: VII, XVII and XX. VII satrapy comprised the people of Sattagydai, Gandarioi, Dadikai, Aparytai; XVII was constituted by Parikanioi and the Aethiopes of Asia, while XX satrapy was that of Indians “the most numerous people of all the men we know”.
The information given by Herodotus finds a precise confirmation in the royal achaemenid inscriptions (DB, DPe, DSe, DNa, DSaa, XPf), whereas the place-names of Thatagus, Gandhara and Hindus are found, as well as the names of the related peoples: one can exclude from this list that of Maka, by someone identified with the coastal baluchi region of Makran (the last Lecocq 1997: 52), and which has been, on the contrary, more properly located in the Arabian peninsula than in the Indo-Iranian frontier (de Blois 1989).
With Hindus it is possible to recognise the middle and low Hindus valley with the exclusion of Gandhara (Bernard 1987: 186), even if the identification of Hindus with the “Indians of the Mountains”, quoted in the late achaemenid epoch texts (Briant 1982: 204) has been, also, proposed.
With Gandhara one can certainly mean the whole course of Kabul river, up to the junction with the Hindus, and not only the Peshawar plain: in the Babylonian and Elamite versions of the inscription of Bisotun, in fact, instead of the place-name Gandhara, one can find that of Paruparaesanna, equivalent to Paropamisadae, with which the classical tradition means the region having as its centre the high valley of Kabul. Interesting is that the city of Kaspapyros/Kaspatyros (cf. Daffinà 1983), which Hecateus collocates in Gandarikè, was associated by Herodotus (III, 102 e IV, 44) to Paktyikè, region contiguous to the northernmost of all other Indians: it is probable that these place-names coincide, and that the second derives from an Iranian etnonym (Herzfeld 1947: 182), even if linguistically non identifiable, in spite of the apparent similarity with the present Pukhtun.
As far as the localisation of Thatagus is concerning, correspondent to the Sattagydia of the Greeks, the opinions of the scholars have been, up to now, discordant (Lecocq 1997: 146-47): recent archaeological investigations in the city of Akra, near Bannu (Khan et al. 2000), would give the confirmation of the localisation of this region in the piedmont area, between Gandhara to North and Arachosia to South, previously proposed (Fleming 1982; contra, Vogelsang 1990: 98, who proposed the region of Multan in southern Punjab). Dadikai seem to correspond to Darada, who in the puranic inscriptions of India are quoted together with Gandharians and Kashmirs (Tucci 1977: 11), while for the Aparytai the identification proposed with the ancestors of the present pashtun tribe of the Afrids (Caroe 1958: 37) should still to be confirmed.
An Indian source of great importance, furthermore, gives a picture of the peoples inhabiting the Indo-Iranian frontier in the 3rd century B.C., so less than one century after the end of the Achaemenid domination: in V edict of the Maurya sovereign Asoka, one has, in fact, a list of peoples amongst which Yona, Kambojas and Gandhara, while in XIII edict is present the composed name of Yonakambojesu. Whether few doubts remain on the identity of Gandharians and of Yona (Greeks of Asia which the Greek version of the edict of the Maurya sovereign, found at Kandahar by the Italian Mission of Is.M.E.O., had been dedicated for), the Kambojas have variously been identified with peoples inhabiting the left side of Kabul river, or better with the peoples which the Aramaic version of Asoka edicts (Scerrato in Pugliese Carratelli and Garbini 1964: 14-15, with bibliography) had been written for: and the linguistic particularities of such a version suggest that they were an Iranian language speaking people (Garbini 1964: 59-61; Bailey 1971).
A paradigmatic example for the Indo-Iranian frontier is that of Arachosia, Harauvatis of the Achaemenid inscriptions, which, according to the representations of the costumes of their inhabitants on the relief of the Achaemenid tombs, seems to be related to other satrapies of eastern Iran, as Areia and Drangiana (Tourovets 2001: 225). Other cultural traits, nonetheless, relate Arachosia to India so much, that its inclusion in the Achaemenid Empire has been interpreted as an Indian penetration to west (Vogelsang 1985). Of particular interest is the appellation of “White India” used by Isidorus of Charax (par. 9), Greek geographer of the 1st century A.D., to describe it (Walser 1985: 154-55). The link of this region of the Iranian plateau to the Indian world is also confirmed, for the following epochs, by a large amount of archaeological evidences. In the Achaemenid inscriptions DSf and DSz from Susa, furthermore, Arachosia is indicated together with Ethiopia and Hindus, as a region famous for ivory, suggesting, thus, that it was a trade-exchange place for the Indian ivory (Vogelsang 1987: 186).