With the decline of the Maury as, India lost its political unity for some time to come. Although the post-Mauryan dynasties are fairly well known, the chronology and territorial extent of most of them is uncertain.
There were foreign invasions too, and a number of regional kingdoms also appeared and disappeared. Foreign elements were assimilated into Indian society as a result of contacts with Central Asia.
In northern India, eastern India, central India and the Deccan, the Mauryas were succeeded by regional dynasties like the Shungas, Kanvas and Satavahanas.
As the successors of Asoka were too weak to resist foreign invasions, in north-western India a number of ruling dynasties from Central Asia established their rule.
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An important cause of the invasions was the weakness of the Seleucid Empire, established in Bactria and the adjoining areas of Iran called Parthia.
The Scythian tribes could not attack China because of the construction of the Great Wall and they began to pressurise the Greeks. The Greeks had no option but to invade India where they did not meet with much resistance.