Darius, the king of Iran penetrated Hindukush and captured Kamboja, Gandhara and Sind in 516 BC. He could do it because:
The region was inhabited by several small principalities like Kamboja, Gandhara and Madra which were constantly fighting each other. There was no central authority.
The area was fertile and rich which attracted evil eyes.
India came in closer contact with West. NW was made a part of Iranian empire and Indian soldiers served in Iranian army.
It began the exploitation at the hands of foreign powers. It had to pay 360 talents of gold ~ 33% of Iranian revenues. Indian soldiers were made to fight Greeks extensively.
It also gave a boost to Indo-Iranian trade and commerce. Iranian coins are found in the area indicating trade relations.
India came in direct contact with Europe. 4 distinct land and sea routes were opened and this increased the Indo-Greek trade and later paved way for Indo-Roman trade.
This led to large umber of Greek settlements in the Afghan region some of which continue till date.
His invasion left detailed historical and geographical accounts. This helps us in understanding the geography of the age and also clearly build Indian chronology. They also tell us in detail about the social conditions like sati, sale of girls by poor parents.
Trade between peninsula and SE Asia had been in existence since 400 BC. Evidences of Indian objects have been found from those sites. Arthasastra and Buddhist literature too talks about Suvarnadeepa.
The initial impetus for this trade came from the Roman trade since Roman demand for spices was quite high which was not available locally in as much quantity. Hence Indians began to import spices from ASEAN. Then from 100 BC the trade picked up basically because of state formations in both lands. And when the Roman trade declined, ASEAN trade increased as it decoupled from the Roman trade. India used to import gold, spices, aromatics, sandalwood, camphor and tin. Exports from India were cloth, cotton, beads, sugar and pottery. The trade was not confined to luxury goods.
Evidences of increased contact are provided by potsherds with brahmi script, carnelian beads etc. in places like the Irrawady delta, the Mekong delta, the Malacca straits, Bali and Java. The prosperity of port towns in India and ASEAN may be linked to this trade.
India continued to import Chinese silk despite learning to grow silk worms here. This is because Indians made silk out of wild silkworms while Chinese silk was fine. Indian brahman merchants also travelled there and established a settlement in Shanghai. Massive expansion of trade took place due to increased Buddhist contacts as well.
Trade with ASEAN continued - now independent of Roman trade as well - and spices and semi precious stones were imported. Malacca, Java, Mekong delta were major centers.
Trade links declined under the Mughals as the Dutch company began to establish its monopoly. Still they used to carry Indian textiles from Coromandel coast to pay for the Indonesian spices.
The evidences from SE Asia show that their early kings often used to trace descent from India in order to derive legitimacy. There is an instance of Cambodian princess marrying a Brahmana (which is said to have introduced Indian culture in Cambodia and thus the story can be seen as an attempt to explain the cultural practices) and an Indian dispossessed king setting upon kingdom in Myanmar.
Many traders for instance from Kalinga are said to have settled in places like Irrawady delta. Indian literature as well as the local literature narrates the adventures of travelers to and fro from these lands. Sometimes the local narratives link the formation of states in ASEAN to the arrival of Indians as traders and as ritual specialists.
Indian buddhists spread Buddhism in China and by late 4th century AD, Buddhism came to be an established religion in China supported by the state. Many Chinese Buddhists came to India on pilgrimage purposes - the most notable being Fa-Hein. Indian elements began to be introduced into the Chinese culture the most visual being the use of Indian techniques in the sculpture and paintings in Buddhist monasteries. Later Buddhism moved from China to Japan.
Growing trade led to settlements and closer interaction. Hindu and Buddhist sculpture and architecture made way in SE Asia and Indian cultural forms also entered the local pattern of life in Cambodia, Thailand, Malacca and Java. The close interaction also assisted in state formation in ASEAN. The process was similar to the state formation in peripheral Mauryan societies except for the heavy dose of caste and thus much of the indigenous customs continued along with the Indian elements. Sanskrit became the language of the elite (influence can be clearly seen in the old Javanese) and in fact some of the finest Sanskrit works come from ASEAN. Many geographical names were kept after Indian places like Ayuthia - the former capital of Thailand. Indian iconographic norms were fused with local forms in the images and Buddhism and Puranic religion mixed with the local religious customs to produce a unique blend. Angkor Vat and the Bayon in Cambodia, the stepped stupa and the Prambanam temple in Java are some of the best manifestations. Local forms of Indian epics came up in ASEAN.
However, ASEAN cannot be referred to as Greater India because the local culture was so visible in the final fused form as well so as to render it a unique form. The Javanese version of the Ramayana retained only the bare bones of the Indian story, the concept of the god king in Cambodia drew from a pre Indian source. The forms of Buddhism in Cambodia and Java were distinctively different from that in India and TIbet. In the court circles only the imitation of Indian things was visible but the rest of the society continued to maintain its own identity after the assimilation of some customs.
Pala kings had close cultural contacts with the Shailendra dynasty ruling over Malaya, Sumatra, Java. Both were Buddhist. The Shailendra king asked for the permission to construct a monastery @ Nalanda and requested the Pala ruler to grant some villages for its upkeep. This was agreed.
The Shailendra king also requested Rajendra I (Chola king) for the creation of a monastery @ Nagapatam and grant one village for its upkeep which was agreed to.
The Chola king, Rajendra I sent a naval expedition against the Sri Vijaya empire in Malay peninsula in order to secure the trade routes to China and also captured some places there.
Embassies were exchanged regularly with China. Examples are Harsha and Chola kings.
The Tibetan king in 7th century encouraged the spread of Buddhism in his kingdom to strengthen the foundations of his kingship as the powerful clans had not yet reconciled themselves with the central rule. He also used it to establish contact with the world beyond Tibet.
Links with Rome
Economic Links
The trade had been in existence since Sangam age. Sangam literature speaks of Roman ships, wine and gold who would exchange it for pepper, textiles and pearls. Roman coins have been concentrated in S India and E Deccan. Few coins are found in W Coast and fewer in NW and Gangetic valley. Romans had a settlement at Arikamedu and Quilon. Towards the later part, there is a clear southward shift in the Roman trade.
In the North and NW, because of the blockade of Silk route in Iran, the Silk route now passed through India - both overland and via sea. Chinese silk became an important item of trade. The trade was now mostly carried in luxury items, not of daily use. Silk was the most important item of export to Rome. Apart from that, spices, muslin, ivory, pearls, precious stones, steel products formed items of exports. The imports were mainly wine, wine-jars, kitchen cutlery, pottery, lead (for Satvahnas), slaves and gold.
It was earlier believed that the trade was mainly carried by Roman sailors because large number of Roman settlements here have been found but not vice versa. At that time the trade was first noticed in the Greco Roman accounts which gave this impression as well. But now evidences indicate substantial Indian presence as well. Given the pre existing trade networks in the peninsula, trade with Egypt and W Asia wouldn’t have been a radical innovation. Potsherds with Indian names inscribed on the in Tamil brahmi have been recovered @ many places in W Asia and Egypt.
Links with Arabs
Cultural
Between 9 - 10 cent AD, numerous scholars from India went to Arab world for knowledge exchange. They taught Indian mathematics, astronomy, medicines to Arabs.
The Rashtrakuta kings let Arabs stay in their kingdom and practice and preach Islam.
Economic
Unlike the Romans, the Arabs settled permanently on the western coast particularly in the Malabar and Konkan region. Thus the Mopilahs are descendants of such settlers. This led to a fusion with local culture and each settling group acquired some customs of the local communities it had close interactions with. Thus there is a difference between the Islam practiced by the Mopilahs of Malabar, the Navayats of Konkan, Khojas and Bohras of Gujarat. In the 7th century, Arab armies invaded Persia and thus some Parsee settlers too came to west coast.
The Rashtrakuta kings had good trade relations with Arabs and even let them settle in their kingdom and practice and preach Islam. They used to import horses from Arabia via Gujarat. Other items of export were pepper and spices along with textiles.
The Pratihara kings also used to import horses from Arabia. The loss of Gujarat meant that they could not replenish the horses for their cavalry which was the mainstay of their army. Other items of export were pepper and spices along with textiles.
Mughals :- The trade relations increased due to growth of 3 empires - Ottoman, Safavid and Mughal. Surat became a major port mainly because of increased Haj traffic, textile production i its hinterland and development of Yemen as a major port.