stylistic:tone
Chandragupta and the Eastern Provinces

Seleucus soon turned his attention once again eastward. The Persian provinces in what is now modern Afghanistan, together with the wealthy kingdom of Gandhara and the states of the Indus Valley, had all submitted to Alexander the Great and become part of his empire. When Alexander died, the Wars of the Diadochi ("Successors") split his empire apart; as his generals fought for control of Alexander's empire. In the eastern territories, Seleucus I Nicator took control of Alexander's conquests. According to the Roman historian Appian:

always lying in wait for the neighboring nations, strong in arms and persuasive in council, he acquired Mesopotamia, Armenia, 'Seleucid' Cappadocia, Persis, Parthia, Bactria, Arabia, Tapouria, Sogdia, Arachosia, Hyrcania, and other adjacent peoples that had been subdued by Alexander, as far as the river Indus, so that the boundaries of his empire were the most extensive in Asia after that of Alexander. The whole region from Phrygia to the Indus was subject to Seleucus.

The Mauryans then annexed the areas around the Indus governed by the four Greek satraps: Nicanor, Phillip, Eudemus and Peithon. This established Mauryan control to the banks of the Indus. Chandragupta's victories convinced Seleucus that he needed to secure his eastern flank. Seeking to hold the Macedonian territories there, Seleucus thus came into conflict with the emerging and expanding Mauryan Empire over the Indus Valley.

In the year 306 BC, Seleucus I Nicator went to India and apparently occupied territory as far as the Indus, and eventually waged war with the Maurya Emperor Chandragupta Maurya. Only a few sources mention his activities in India. Chandragupta (known in Greek sources as Sandrokottos), founder of the Mauryan empire, had conquered the Indus valley and several other parts of the easternmost regions of Alexander's empire. Seleucus began a campaign against Chandragupta and crossed the Indus. Most western historians note that it appears to have fared poorly as he did not achieve his goals, even though what exactly happened is unknown. The two leaders ultimately reached an agreement, and through a treaty sealed in 303 BC, Seleucus abandoned the territories he could never securely hold in exchange for stabilizing the East and obtaining elephants, with which he could turn his attention against his great western rival, Antigonus Monophthalmus. The 500 war elephants Seleucus obtained from Chandragupta were to play a key role in the forthcoming battles, particularly at Ipsus against Antigonus and Demetrius. The Maurya king might have married the daughter of Seleucus. According to Strabo, the ceded territories bordered the Indus:

The geographical position of the tribes is as follows: along the Indus are the Paropamisadae, above whom lies the Paropamisus mountain: then, towards the south, the Arachoti: then next, towards the south, the Gedroseni, with the other tribes that occupy the seaboard; and the Indus lies, latitudinally, alongside all these places; and of these places, in part, some that lie along the Indus are held by Indians, although they formerly belonged to the Persians. Alexander took these away from the Arians and established settlements of his own, but Seleucus Nicator gave them to Sandrocottus, upon terms of intermarriage and of receiving in exchange five hundred elephants. — Strabo 15.2.9

From this, it seems that Seleucus surrendered the easternmost provinces of Arachosia, Gedrosia, Paropamisadae and perhaps also Aria. On the other hand, he was accepted by other satraps of the eastern provinces. His Persian wife, Apama, may have helped him implement his rule in Bactria and Sogdiana. This would tend to be corroborated archaeologically, as concrete indications of Mauryan influence, such as the inscriptions of the Edicts of Ashoka which are known to be located in, for example, Kandhahar in today's southern Afghanistan.

Some authors say that the argument relating to Seleucus handing over more of what is now southern Afghanistan is an exaggeration originating in a statement by Pliny the Elder referring not specifically to the lands received by Chandragupta, but rather to the various opinions of geographers regarding the definition of the word "India":

Most geographers, in fact, do not look upon India as bounded by the river Indus, but add to it the four satrapies of the Gedrose, the Arachotë, the Aria, and the Paropamisadë, the River Cophes thus forming the extreme boundary of India. According to other writers, however, all these territories, are reckoned as belonging to the country of the Aria. — Pliny, Natural History VI, 23

Nevertheless, it is usually considered today that Arachosia and the other three regions did become dominions of the Mauryan Empire.

The alliance between Chandragupta and Seleucus was affirmed with a marriage (Epigamia). Chandragupta or his son may have married a daughter of Seleucus, or perhaps there was diplomatic recognition of intermarriage between Indians and Greeks. As well, an Indian Puranic source, the Pratisarga Parva of the Bhavishya Purana, also described the marriage of Chandragupta with a Greek (" Yavana ") princess, daughter of Seleucus (Suluva in Indian sources).

In addition to this matrimonial recognition or alliance, Seleucus dispatched an ambassador, Megasthenes, to the Mauryan court at Pataliputra (Modern Patna in Bihar state). Only short extracts remain of Megasthenes' description of the journey.

The two rulers seem to have been on very good terms, as classical sources have recorded that following their treaty, Chandragupta sent various presents such as aphrodisiacs to Seleucus.

Seleucus obtained knowledge of most of northern India, as explained by Pliny the Elder through his numerous embassies to the Mauryan Empire:

The other parts of the country beyond the Hydaspes, the farthest extent of Alexander's conquests were discovered and surveyed by Seleucus Nicator: namely

Seleucus apparently minted coins during his stay in India, as several coins in his name are in the Indian standard and have been excavated in India. These coins describe him as "Basileus" ("King"), which implies a date later than 306 BC. Some of them also mention Seleucus in association with his son Antiochus as king, which would also imply a date as late as 293 BC. No Seleucid coins were struck in India thereafter and confirm the reversal of territory west of the Indus to Chandragupta.

Seleucus may have founded a navy in the Persian Gulf and in the Indian Ocean.