One thing any serious student of ancient history will notice when he travels the length and breadth of India is the lack of true Kshatriyas descended from the imperial bloodlines mentioned in the Vedas, Epics and Puranas. On the other hand - Brahmins, Vaisyas and Shudras are found nearly everywhere, and well-represented in the historically high density population regions.
All Kshatriya castes in India can be classified into three categories:
1) Recent claimants to Kshatriya identity e.g. Ahirs, Kurmis, Reddys, Marathas through a process of mythologizing and sankritization
2) Descendents of the 4th -7th century Scythian hordes whom Brahmins converted en masse to the neoVedic religion e.g. Rajputs, Gujjars
3) Castes with somewhat more credible claims to true Kshatriya status e.g. Khatris (word itself a derivative of Kshatriya, located mainly in SaptaSaaraswat), Jats (widely considered Irano-Scythian in origin, but with a long history dating back to at least Harshavardhana, whom everybody considered to be a Kshatriya and and possibly even to the Sacrifice of Daksha Prajapati).
Even if the Khatris' and Jats' somewhat tenuous claims on being the descendants of the imperial Kshatriyas are upheld, that leaves huge stretches of Bharata Varsha bereft of Bharatas. Particularly galling is their absence in the Gangetic plains, which nurtured the later stage of Indo-Aryan civilization.
Now if we look to literary sources for clarification, we know from the ancient Battle of Ten Kings in RV and other sources that the Trtsus, Purus, Yadus and Matsyas got to stay in the SaptaSaaraswat out of the initial grouping of Aryan tribes while Pakthas (Pakhtoons), Parsus (Persians), Parthas (Parthians), Bhargavas (Phrygians), Drhyus, Anus, Bhalanas (Alans ?), Panis and Dahae (Scythians ?) were driven out. We also know that Trtsus and Purus merged to form the mighty Bharatas, who left their indelible mark on the culture and civilization of what passes today as Hindustan.
From Chanakya's Arthashastra and Megasthenes' Indica, we know that some vestiges of the regal Kshatriyas were present in 300-200 BC since one of Chandragupta Maurya's rivals was the ruler of Punjab, a descendant of Porus and presumably a Bharata by blood. Going back further, Buddha is supposed to have been a Kshatriya. But reading between the lines, one can see that even at that time, the population of Kshatriyas was very small compared to the other three castes, strengthening the argument for some type of holocaust at an early time.
There are at least two Kshatriya holocausts mentioned in the ancient texts:
1) Parashuram (Persian Rama, descended from the Phrygian emigre Jamadagni) carried out multiple exterminations, particularly of the Yadus since the Hehayas who killed his father belonged to this tribe. Many Kshatriya tribes are said to have left India during this period. Because of the outsized role of the Bhargavas in redacting these myths, scholars take the Bhargava annihilation of Kshatriyas with a grain of salt.
2) Kurukshetra: Internecine warfare between royal Kshatriyas resulted in complete destruction of several Kshatriya races. Only the Yadavas, who had diminshed status compared to other Kshatriyas (could not assume Kingship), survived the Mahabharata War. (Yadavas too are said to have perished at Dwaraka subsequently but there is no evidence for their destruction elsewhere in India). The epic itself puts the death toll at 4 million. If taken at face value, this would be one-fourth the horror of WWI and one-thirteenth that of WWII.
The key issue here (also a vexing one) is dating the Mahabharata War, assuming it happened and dating Parashuram, assuming he existed.
What has been left un-researched is the possible socioeconomic consequences of such a disastrous war in a pre-modern society. One could predict hundreds possibly even thousands of years of peace as the most aggressive members of society were no longer present but also stasis as leaders, men with initiative and risk takers disappeared from society .
The burden of supporting the lavish and wasteful lifestyle, and war-debts of the Kshatriyas no longer had to be borne by the working classes, resulting in their liberation from tyranny, oppression and exploitation (hence the elevation of Krishna Vasudeva who was the architect of the Mahabharata War to a God).
Parasitical priests (Brahmins) previously reliant on the largesse of Kings during ostentatious sacrifices, war rituals, funereal rites, etc. were forced to impose the burden of paying for their services on an unwilling population, eventually resulting in the birth of anti-Brahmin populist movements such as Buddhism and Jainism, which sought to use the principle of "Ahimsa" and vegetarianism to destroy the very root of Brahminical power namely the Yagna or animal sacrificing fire ritual (with mixed results).
@padamchen @Joe Shearer @niaz