As in OTL, "India" is a not a term denominating a single country, a linguistically, religiously, ethnically or even just spatially clear-cut entity. It is more of a historical and political schema: The concept has several cores (none of which absolutely defines it) and fuzzy boundaries. This concerns the space occupied by "India" as well as its ethnic groups, its religion, culture, socio-economic structures and dynamics, and its polities.
In this sense of the word, "India" becomes larger, growing mostly in an Eastward direction in the time span between 250 and 750, both in OTL and in this timeline.
Beyond these similarities, there are major differences between India`s developments in OTL and in this timeline.
Trade and Contact with Europe
In OTL, European-Indian trade diminished throughout the 4th to 6th centuries, due to the downfall of the Roman Empire and the de-urbanisation of the Mediterranean economy and society. In the 7th century, it was radically reduced as the Islamic conquest created a new barrier. As a result, India de-urbanised, too, and entered its own rural Middle Ages, complete with urban "shreni" (guilds) as conservative forces, where technology is preserved, not revolutionised.
In this timeline, Roman economy develops very dynamically. Trade volumes with similarly prosperous India remain high. A comparatively peaceful Sassanid Empire, and its successor state, Mazdakite Eran, as well as the Christian Kingdom of Sheba, which during this period gains control over the entire Arabian peninsula, keep trade routes open. Technological innovations from Rome (textile mills, gaffering, fire syphons) as well as Persia (windmills) reach India, just like Indian steel-working techniques and Devangari numerals spread to the West.
As a result, urban middle classes of craftsmen and traders are strengthened in India. Their manufacturing enterprises face sharp competition from the West, though, which inspires innovation in India, too.
On India`s coasts, various Roman and Ostrogothic emporia (like Cochin) develop and grow, while large Indian port towns become even more multicultural than they already were.
Trade contacts with Europe exert influences on political, military and religious developments as well.
Sassanid Age vs Gupta Age
In OTL, Northern India`s centralisation happened under the reign of the Guptas, who conquered manifold polities in Northern India and reformed and consolidated existing Indian cultural and socioeconomic traditions and developments.
In this timeline, centralisation in North-Western India as far as the Yamuna occurs under Sassanid rule until the 6th century. Its Indian satrapies are grouped into Kushana (in the Far West) and Sindhu-Shakastan (immediately to Kushana`s East).
Kushana was conquered by the Sassanid Empire in the 240s and kept a large degree of autonomy.
The satrapies of the Sakas had been under Kushan suzerainty and experienced a short period of independence under Rudrasena (250s-270s). As the Sassanids had completed their conquests in the West and North, they focussed on exacting tribute from the Sakas. Bhartrdaman put up resistance, and Shapur II. sent his huge Sassanid army onto the last campaign of conquest under his reign, defeating Bhartrdaman and installing a Sassanid satrap of Sindhu-Shakastan.
After several revolts in Sindhu-Shakastan, the Sassanids involved themselves more directly in controlling their Indian territories: land which previous rulers had given to Brahmins (brahmadeya) was confiscated and given to azatan from the heartland, who formed the backbone of the new Sassanid army, nobility and regional administration which controlled Sindu-Shakastan. Brahmins lost their powerbase and continued to put up helpless resistance, which often led to their deportation.
Brahmadeya cemented the social power of the Brahmins and the weakness of Indian states in OTL. In this timeline, it is limited to a belt from the Deccan to tha Ganges delta, where several weak dynasties contend with each other for power.
During a short period of religious perseuction instigated by Kartir, martial arts societies developed around Buddhist monasteries, but also among marginalised Brahmins. After a period of religious persecutions instigated by Kartir, religious tolerance came back to Sassanid India in the 4th century, with Iranian influences trickling in instead of being imposed.
The Sassanids defended their Indian possessions in the 4th century against the Vakatakas under Pravarasena I. (310s) and against the Guptas under Samudragupta (350s), reducing both empires, which are large in OTL, to mere regional players. Under Sassanid rule, craftsmen, traders, landowners and bankers (many of which followed old and new Buddhist schools or even observed Zoroastrian cult) become the new powerful force in India's society. Among the poor majority of peasants and contractual workers, affordable food and clothing and access to clean water reduce child mortality and increase life expectancy. Their popular beliefs in various deities persists unaffectedly, but does not undergo the synthesis processes occurring during this period in OTL Gupta and Vakataka India.
The Sassanids defended their Indian conquests against the Gupta and Vakataka Empires, which had formed an alliance against the Sassanids. After the fortification of the Sassanid Empire's Eastern border, the empire's Indian possessions were not militarily challenged for two centuries.
Within the Sassanid Empire, Kushana and Sindhu-Shakastan contributed greatly to the empire's wealth. North-West India's integration into the booming and innovative Sassanid economy brought technological innovations like windmills, mechanised cotton carding, glass-blowing over oil lamps and many more to the land between Indus, Mahi and Yamuna. Dams and canals for irrigation and the use of hydromechanical power are built. The population of the Sassanids' Indian satrapies grows - and it grows quicker than that of the rest of India.
Sassanid India is also the place where Western and Indian theories and sciences meet and fertilise each other (once again, after an earlier peak of cross-fertilisation in the 4th century BCE). Indian mathematics become known and are adapated in the Middle East and Europe. In the 5th century, Indian Carvaka empiricism and Roman / Alexandrian empiricism cross-fertilise and strengthen each other, facilitating the establishment of differentiated natural sciences at universities like Barygaza. A century later, the Indian Samkhya tradition and Italian Rationalism would cross-fertilise each other.
The latter are also centres for the dissemination of the Pahlavi language and alphabet, which coexists during Sassanid rule with Prakrit languages written in the Brahmi script.
In the absence of imperial Gupta conquests, a great number of ethnic groups especially in the Himalaya remains relatively unassimilated and continues its autonomous existence throughout the first millennium CE: Kinnara, Lomosa, Karavapana, Dasa, Kanghra, Nalagarh, Kuninda and various smaller Khosa groups.
When the Sassanid Empire collapses in the early 6th century, Kushanshah Vasishka II. successfully declares independence. In Sindhu-Shakastan, conflicts between emerging city republics, warring tribes and defenders of the old order endure, but Vasishka manages to include a growing number of polities voluntarily into his realm, where they continue to enjoy a great degree of autonomy and must keep peace with each other.
After four decades of renewed glory, Later Kushana suffers a defeat at the hands of the Göktürks. Taxila, its capital, is destroyed. Meeting continuously fierce resistance, the Göktürks withdraw. After the fall of Kushana, the North-Western Indian cities form the "Shaihr Jaari", a federation for mutual protection in which the Kabul Shahis play a vital and new role as a very pious Buddhist warrior group, which does not seek imperial control, but offers its protection to a growing group of cities and remains open for young peasants to join their ranks for a very long time. In the Sindh, the Abhira assume a similar role for the city republics there.
From the 7th century on, all of former Sassanid India has become a patchwork of criss-cross-allied city republics and small kingdoms. Tribal chiefdoms in the Himalayas, which in OTL were subdued and Hinduised by the Gupta Empire, remain untouched.
The Gupta Empire, on the other hand, was smaller than in OTL, but outlived its OTL equivalent by far, gradually dissolving into city republics (gana sanghas) instead of being destroyed by invading Hephtalites. The Roman - and Sogdian, and Swahili - model of independent city republics became so attractive for the urban middle classes and their strong economic associations that much of Northern India came to be controlled by city republics in the 6th, 7th and 8th centuries (and long after that, too). The city republics formed alliances, which spanned across the boundaries between what had been Sassanid India and what had been Gupta India. This - and other factors like unable Gupta emperors - led to the transformation of imperial Gupta rule into a very loose framework for highly autonomous polities, not unlike OTL´s Holy Roman Empire.
The international Silk Road Protocol, which created the necessity of armed convoys, which in India were organised by allowing the guilds to maintain small armed forces which protected their trade caravans as well as the city republics, only corroborated this structure.
Tamilakam: No Hegemony of Indo-Aryan Influences
Indian Democratic Theory and Practice: the "Gana Sangha"
India had a great republican tradition in the 1st millennium BCE. By 250, it was already weakened and had increasingly been replaced by monarchies - a process to which Chinese models or analogous processes in Persia and the Roman Empire may have contributed.
In OTL, this erosion of republics continues and their replacement with empires and their highly hierarchical structures (a maharaja, an administration from among the nobility on several levels, local autonomy under the leadership of "village elders", and across this stratification another social one in the form of the varnas (castes).
In this timeline, Western influences as well as socioeconomic dynamics take a different turn, beginning with the democratic Roman Revolution. While republics had persisted at the fringes of the empires, from the 5th century on, more and more large city republics establish themselves in fierce struggles for independence. In the 6th and especially in the 7th century, they form (military, economic and political) alliances, the most important among them the Shaihr Jaari, formed in Kushana, and the Kannauj Alliance forged by the republican politician Harsha (who in OTL becomes an Emperor instead). The political philosophical foundation was laid by the 5th century philosopher Bhaskara and others, who brought forth a group of texts named Ganatantrashastras.
Most Indian republics (gana sanghas) were more oligarchic than democratic, admittedly. In assemblies, town councils or administrative councils, the speakers of guilds (shreni in the North, nikamam in Tamilakam), the leaders of religious groups and the heads of important clans (in the North kshatriyas) usually met, while not only women, but also younger, less wealthy, influential or educated men and shudras were excluded. Indian republican political philosophy was controversial on this matter: both oligarchic and egalitarian views were elaborated throughout the 6th to 10th centuries, rekindling the interest in older Indian philosophies as well as absorbing contrasting influences from the Mediterranean, Iran and China.
Indian society is conservative - both in OTL and in this timeline. The main reason behind this is the strength of social structures and the weakness of states. While in OTL, this brought forth weak warrying monarchies, in this timeline, Indian statehood is mostly non-aggressive and interwoven even more into the social fabric of traditional rules. The importance of consensus decisions in many gana sanghas reflects this.
From the 7th century on, popular revolts threatened the oligarchical order and installed egalitarian republics in some places. They had economic causes, often - as in the case of the 650s - they were escalated debt crises. India`s gana sangha system survived for another at least three centuries because it adapated to this challenge, too, establishing a modus vivendi between oligarchies in wealthy port towns and egalitarian republics in other places. The introduction of regular Jubilees increased socioeconomic and thus also political stability.
It is this mixed bag of monarchies, oligarchies and democracies which India exports into South-East Asia in this timeline, instead of exclusive monarchism as in OTL. Thus, Nusantaran (Indonesian), Mon and Pyu city states also show a greater variety of political constitutions.
India`s Guilds: Stability and Innovation
Guilds (shreni; nikamam) shaped India`s economy both in OTL and in this timeline. In both, they provide social cohesion, social security and stability, maintained professional standards and kept urbanism alive in the post-classic period.
In OTL, they functioned in a framework of rigidifying varnas and jatis. They protected secret knowledge, they were tightly closed groups, they even forbade intermarriage. On the other hand, they stayed out of "big" (i.e. imperial) politics.
In this timeline, Roman competition and different religio-cultural developments (a stronger Buddhism and the retardation of the "Hindu synthesis" as well as its happening under more intense foreign influences, see Sassanid vs. Gupta India) bring forth a different path of development for India`s guilds. Under Sassanid rule, they are among the few legally recognised Indian civil associations, yet many guilds train their members in martial arts so as to be able to defend themselves against oppression. After the breakdown of the Sassanid Empire, North India`s shreni take over political control, forming city councils of city republics, knitting inter-city alliances and paving the way for the multi-dimensional network of allegiances which kept Greater India at peace for more than four centuries (7th to 11th)
Economically, the guilds had to adapt and open themselves towards innovations, lest they be outcompeted by cheaper Roman textile, weapon and glass manufacturers and advanced Chinese porcelain manufacturers. India`s guilds often pressed for protectionist policies, but they were also the main force which helped society to manage such transformations, e.g. by providing for its members in times of crises and by accessing investment capital due to their trustworthiness.
Religion and Culture
In OTL, this time span marks the decline of popular Buddhism and its marginalisation as a merely monastic cult. At the same time, Puranic Hinduism emerged dominant on the entire subcontinent, bridging brahmanic and popular cults and reinterpreting endless local deities as avatars of a canonical Hindu pantheon.
This was linked to three major developments: invasions from the West and North-West (Hephtalites, later Muslims), the establishment of large religiously orthodox (Astika) empires (Gupta, Vakataka, later Chalukya and Pallava) and a tendency towards a static social order, which outlived the large empires.
In this timeline, India is not invaded by Muslims or Hephtalites, the Vaishnavist Guptas and the Vedic-orthodox Vakatakas beginning with Pravarasena establish only middle-sized empires, which are weaker than their Sassanid neighbour in the West, which influences religious traditions, too. Also, the increased contacts with the powerful Roman economy do not allow for socioeconomic stagnation.
Hindu schools of thought, especially Carvaka, Samkhya and Mimamsa, blossom and cross-fertilise with Mediterranean philosophical schools. While Carvaka has always been atheist, the other two schools of thought separate themselves from the theological domain, too, during this period in this timeline. In the 8th century, Mimamsa philosophers contribute to the formation of the separate scientific discipline of linguistics, which quickly becomes popular in the Mediterranean, too, where grammar had always played a prominent role.
The Puranas, although not non-existent, do not experience a wide dissemination across the entire subcontinent because only Eastern India stil has traditional brahmins who, in OTL, are responsible for the mainstreaming of local religious traditions across the subcontinent.
The Bhakti movement originates in the North (instead of in the South as in OTL) and eliminates vedic cult altogether. A much larger variety of deities is worshipped in this personal way, and their identification with Nirguna Brahman is not pervasive.
No child named Adi Shankara is born because Kerala is not Hindu-synthesised in this timeline. Smartism does not even develop. Instead, Tamil religious views and practices, which are essentially animist and center around fertility and female figures, experience a very different systemisation through Roman and Mazdakist Iranian influences.
Buddhism remains a popular religion on the entire subcontinent and expands in all directions. Heavy debt crises threaten the Theravada, Mahayana and Vajrayana monasteries, though, whose monks do not work and depend on their function as main lenders in order to finance themselves. In this context, Dhyana (OTL Zen) Buddhism, whose monks meditate through work and martial training, too, gains influence in India and has achieved religious hegemony by 750 CE.
India`s Influence, or Greater India
Both in OTL and in this timeline, Indian influences reach far beyond the subcontinent, mostly in a peaceful manner. Religiously, Hinduism and Buddhism spread across much of East Asia, Sanskrit and/or Pali loanwords are found in languages like Burmese, Thai, Khmer, Indonesian etc.
In OTL, India exported the political models of monarchy and rajamandala to South-East Asia. Towards the West and North-West, Indian cults were retreating and Islam spread.
In this timeline, the polities on the Indian subcontinent are either city republics or small kingdoms, among which a complex network of criss-crossing alliances spanned, which kept the region at peace and its many small statelets independent. This cultural model is one of India`s export success stories in the second half of the first millennium CE: It is copied in the Pyu Alliance, the Dvaravati Alliance, in Sri Gotapura, by Kedah, Pan Pan, Beruas, and Singhapura on the Malay Peninsula, by Jambi, Kantoli and Palembang on Suvarmadvipa (OTL Sumatra), by Kalingga, Sunda and Tarumanaggara on Java, by Kutai and Po-ni on Borneo, and by Marwagarh and Tipukolu at the mouth of the Limpopo in South-West Africa. The Iranian-speaking Sogdian Federation and the cities along the Oxus and in Bactria followed similar models, although it is unclear as to whether this is due to Indian influences.
In this timeline, Islam never develops into a world religion, and Buddhism remains firmly anchored all along the Silk Road in Central Asia and even exerts influence in the Northern Caucasus and on Turkic and Samoyedic tribes.