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I have to contest some of this.
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As far as I know, China was not forced back together due to steppe pressures. The steppe was not a military issue until after the Bronze Age, when Chinese civilization was already developing a somewhat unified government model in the Central Valley. The Xia and Shang of the Middle and Late Bronze Age, had little to no difficulties with their steppe neighbors. In fact, they were improved by them through trade. Evidences that come to light, is that the Chinese Xia and Shang Dynasty were active partners in the trade network between Europe, China, Elam-Mesopotamia and the Indus. With semi-sedentary Indo-European peoples, the Oxus Civilization and the Elamites serving as intermediates, many things arrived in China. Bronze making from the west, the chariot/wheels, different sorts of crops and materials for wealth, jade, gold, amber and so forth. Only whence the Chinese culture was increasingly centred around unified monarchies and untied by the 9 Cauldrons of the Xia, did the steppe nomads to the north develop new sorts of bows and breed stronger horses for which to pose a threat to the people of the Central Valley, this occurred around 950-900 BCE in the northwest of the Chinese Central Valley; wherein western innovations possibly of the Scythian-Saka and others, developed more fearsome and aggressive modes of warfare than the old chariot warfare of the Middle Bronze Age. The Zhou countered these nomads very poorly, simply chasing them across the border and then returning. These steppe armies would remain a nuisance until the formation of a true steppe empire during the reign of Maodu of the Xiongnu in the Early Western Han.
During the formation of the Chinese civilization sphere, we could argue that China had a fairly ideal situation, all things considered. Its south was inhabited by jungles and mountains, less populated than their western counterparts in Mesopotamia and their north was a vector of trade and material improvement. The region that comprised the Aryan states, was not so.
For whateevr reason, the IVC collapsed around 1950 BCE and the agriculture of the land was dry and difficult. No thanks to the excessive drying and famines common from 2100-1900 BCE across the west at that time. The arrival of Indo-European peoples into the Indus, came upon a collapsed society and as the Vedas mention 'Veda destroys agriculture and agriculture destroys Veda.' The Vedic period composed a series of pastoral societies from the Indus to the Bengal until a period of centralization, which centralized along the lines of Aryan kinship and lineage groups forming into kingdoms, republics and oligarchies. This was a society that had no framework for a united entity and state, as the Central Valley of China did to its ancient Xia predecessors did. Comparing the Magadhi and the Aryan states to China is like comparing ancient Mycenaen Greece or the Hittite confederate kingdoms to the Ancient Egyptian realm, the Kingdom of the Two Horizons.
China's geopolitical situation was also better once it reached mature phase than the Gangetic Plain or the Aryan range of civilization. The Chinese had really only one offensive threat, that being their north and northwest, with the northwest taking the greatest precedence. Dangers from the Southern nomads had been remedied in the Zhou period to a large degree, with the creation of the states of Chu, Wu and Shu. The true east, held Japan and Korea, which were non-issues to Chinese stability in this period.
Standing in the way of the Aryans is not only the difficulty in uniting the Aryans, but also a complex and dangerous geopolitical sphere. As I described earlier, Hindu geopolitics played as a series of movements and reactions. Powerful states and peoples in the west move into the subcontinent and battle for hegemony, the Aryan states unite and attempt to unite the subcontinent and against both sides, a southern state arises in the Deccan that opposes both hegemonies and exists as a wrench in hegemonic formations in the subcontinent. China experienced nothing to this level. Magadhi hegemonies when they formed, were always fighting simply to survive against offensive and powerful realms, even after they have united the Gangetic Plains. China would have no foes except to its northwest and northeast after uniting its Central Valley. The fact that the Chinese lacked a sufficient southern offensive foe in the south, is what possibly permitted the Han to more efficiently defeat the Xiongnu. Had they not had this geopolitical situation, it is likely that they would have been much more hard pressed to maintain their Central Valley's integrity.