Despite the references to the deities as asuras, towards the end of the Samaveda, Surya has been praised as the killer of vritra, dasyus, asuras and foes, while Indra has been praised for overthrowing the great might of the asuras. It wud mean that the rig and the early sama still carried the positive connotation of asura, but the asuras became foes in the late-sama and in the yajur veda.
The fight b/w the devas and asuras (now an inimical set) shows up in the yajur explicitly as seen in the text in this link:
Yajur Veda Kanda I - it seems, it was about who got control of Agni. According to the Krishna yajurved, at one point, by deceit, the devas took away the wealth of the asuras and drove them away. So cud it be possible that the asuras at this point of time moved into south into Southern India?? There are also quite a few references to the asuras being driven away in the Yajur.
This is from the YV (taittriya) Kanda 2 abt the devas (gods) and asuras (the break-away faction) at conflict with one another. It is quite revealing. They had some sort of a re-organization, and finally accepted Indra as the leader (Krishna YV 2.2.11):
The gods and the Asuras were in conflict; the gods were mutually at variance; unwilling to accept the pre-eminence of another they went apart in four bodies, Agni with the Vasus, Soma with the Rudras, Indra with the Maruts, Varuna with the Adityas. Indra had recourse to Prajapati; he made him sacrifice with the verse for harmony; to Agni with the Vasus he offered a cake on eight potsherds, to Soma with the Rudras an oblation, to Indra with the Maruts a cake on eleven potsherds, to Varuna with the Adityas an oblation; then indeed the gods agreed to recognize Indra's pre-eminence.
Imo, the story of devas and asuras in conflict in puranas is a continuation of their fight in the Vedas. Both the groups either exiled the other on winning or made the other as slaves. This has been a typical scenario with the Indian kingdoms even ages later. The winning king either exiles the defeated king (desha brashtam) or enslaves the defeated group. The defeated group recoups in time and defeats its aggressor; and the saga keeps repeating. The frequent fights b/w devas-asuras i think is a continuum from the Vedas into the Puranas.
The gods and the Asuras were in conflict..the Asuras conquered the gods, the gods being defeated became the servants of the Asuras; from them power and strength departed; Indra perceived this; he departed in pursuit of it; he could not win it. - Krishna VY 2.3.7
The Devas, Manavas, and the Pitrs were on one side, while the Asuras, Raksases, and Piçacas on the other….Then indeed did the gods conquer the Asuras, and having conquered the Asuras, they drove away the Raksases. - Krishna YV 2.4.1
In KYV.2.4.3, I found it quite comical that the devas and asuras both hailed Gayatri, but they sought her blessings in rivalry of each other (power struggle). And the devas managed to please Gayatri and appropriated the force, might, power, strength, offspring, and cattle of the Asuras.
In the Krishna YV there still are links b/w the devas and asuras. Viçvarupa, son of Tvastr, was the domestic priest of the gods, and the sister's son of the Asuras. But Indra smote off his heads. The creatures called Indra “Thou art a Brahman slayer”. Then Indra appealed to the earth, 'Take a third part of my guilt.' Indra seems to have gone any length to keep power for himself. Here is a link mentioning that Devas married daughters of Asuras:
Biographies of Asvagosha, Nagarjuna, Aryadeva, and Vasubandhu
Of all the Vedas, perhaps the Yajur holds the key to explaining how the differences b/w the dEvas and asUrAs came about.
Am not sure the squabble b/w the Indo-Iranians was about monotheism and polytheism. It seems to me that it is was that too, however it was more of a power struggle (or political struggle) b/w factious groups (it wud be like the numerous factions of the Congress or Janata dal in India, each becoming allies at one time or foes at an other time).
Some ppl opine that the asUras were the AdityAs in a power struggle against their step-siblings while others opine that the dEvas were the adityAs up against the asUrAs.
From what I have read so far, Indra, Varuna and Mitra seem to be invoked (praised) quite often in all the Vedas (sometime a deity like Parjanya is found in atharva but am told is not present in the yajur veda). Shri Saidevo had mentioned that the asuras had no varna divisions (I forgot which thread). But then Raavan was an asura and a brahmana (compare this to the Assyrian king, Ashur bani pal who was also a priest).
I do not think there was varna classification in the early vedic period. I think it came about only later. Some semblence of it showed up from the Yajurved period. Imo, the vedic classes or groups of people were majorly devas and asuras, who possibly later branched into further divisions like yakshas, gandharvas, etc…and perhaps then later into further branches like darada, kambhoja, pulinda, etc. This is a nice reading on the Deva-Asura matrix:
Indian Myth and Legend: Chapter IV. Demons and Giants and Fairies
These are some interesting notes I wanted to make:
Referring to the vedic metres:
Prajapati created the gods and the Asuras; thereafter the sacrifice was created, after the sacrifice the metres; they went away in all directions, the sacrifice went after the Asuras, the metres after the sacrifice; the gods reflected, 'These have become what we are'; they had recourse to Prajapati; Prajapati said, 'Taking the strength of the metres I shall bestow it upon you.' He took the strength of the metres and bestowed it upon them. Then the metres ran away, and the sacrifice followed the metres. Then the gods prospered, the Asuras were defeated. – KYV 3.3.7
Indra, is mentioned as the son of Ekastaka:
The Ekastaka, undergoing penance, Hath borne a child, the great Indra; Therewith the gods overpowered the Asuras; Slayer of Asuras he became in his might. – KYV 4.3.11
Shukla YV 40.2 and 40.3 showing up its wrath against the asuras wrt to performance of karmas:
One, only doing Karma here, should wish to live a hundred years. No way is there for thee but this. So Karma cleaveth not to man. Aye, to the Asuras belong those worlds enwrapt in blinding gloom. To them, when life on earth is done, depart the men who kill the Self.
Krishna Yajur Ved 6.2.3:
The Asuras had three citadels; the lowest was of iron, then there was one of silver, then one of gold. The gods could not conquer them; they sought to conquer them by siege… Therefore is Rudra overlord of animals. Rudra let it go; it cleft the three citadels and drove the Asuras away from these worlds.
I found this highly intriguing
Yajur Veda Kanda VI :
The sacrifice went away from the gods in the form of Visnu, and entered the earth. The gods sought him grasping hands. Indra passed over him. He said, 'Who has passed over me?' 'I am he who smites in the stronghold; who art thou?' 'I am he who brings from the stronghold.' He said, 'Thou art called he who smites in the stronghold. Now a boar, stealer of the good, keeps the wealth of the Asuras which is to be won beyond the seven hills. Him smite, if thou art he who smites in the stronghold.' He plucked out a bunch of Darbha grass, pierced the seven hills, and smote him. He said, 'Thou art called he who brings from the stronghold; bring him.' So the sacrifice bore off the sacrifice for them; in that they won the wealth of the Asuras which was to be won (védyam), that alone is the reason why the Vedi is so called. The Asuras indeed at first owned the earth, the gods had so much as one seated can espy. The gods said, 'May we also have a share in the earth?' 'How much shall we give you?' 'Give us as much as this Salavrki can thrice run round.' Indra taking the form of a Salavrki thrice ran round on all sides the earth. So they won the earth, and in that they won it therefore is the Vedi so called.
Krishna YV 6.3.9:
The Viraj dividing itself stayed among the gods with the holy power (Brahman), among the Asuras with food. The gods desired, 'May we acquire both the holy power (Brahman) and food! They saw (the rite of) these twenty nights. Then indeed they acquired both the holy power (Brahman) and food.
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Struggle for purohitship began in the Krishna Yajur Ved - what could have been the reason then ? Imo, we might have to look into the role of the yajurvedi in creating the dharmashastras from this period onwards :
Yajur Veda Kanda II
He who has a dispute for a Purohitaship should offer (a beast) with a black neck to Agni, a brown one to Soma, and one with a black neck to Agni; the Brahman is connected with Agni, the prince with Soma; on either side of (the beast) for Soma there is one for Agni; verily with brilliance, with the Brahman, he seizes on either side the kingdom, and forthwith appropriates it; they choose him as Purohita. The gods and the Asuras strove for these worlds; Visnu saw this dwarf, he offered it to its own deity; then he conquered these worlds. One who is engaged in a struggle should offer a dwarf (beast) to Visnu; then he becomes Visnu and conquers these worlds.....
If a Brahman and a non-Brahman have a litigation, one should support the Brahman; if one supports the Brahman, one supports oneself; if one opposes the Brahman, one opposes oneself therefore one should not oppose a Brahman.