Why didn't Kṛṣṇa really prevent the Mahābhārata war? Did Kṛṣṇa classify the Kurukṣetra war as hiṁsā or ahiṁsā?

Question

1. Why didn't Kṛṣṇa really prevent the Mahābhārata war from taking place? Was the war really worth it? This answer says the war resulted in the death of more than a billion soldiers, horses and elephants on both sides. The numbers maybe inflated but what a great calamity! Isn't it the biggest war ever to take place on the face of earth? Why show viśva-rūpa to Arjuna and convince him to raise his Gāṇḍīva against his own cousins but why not show the same viśva-rūpa to everyone in the battlefield and stop the war?

   Oft-cited explanations are: It's all Kṛṣṇa's līlā. Kṛṣṇa wanted to "wipe out the bad kṣatriyas" or "reduce the burden on Earth." Really? I don't buy it. It's like saying: let's kill all the old people on earth ('burden'), let's kill all the diseased ('burden'), let's bomb the whole building because there's a terrorist hiding in the basement ('lose some to gain some')!

2. I think Gāndhārī asked Kṛṣṇa the same question (in the title); what was his response to her? Was she fully convinced with the answer he gave?

3. What exactly is Kṛṣṇa's view of hiṁsā or ahiṁsā? When to use hiṁsā and when to use ahiṁsā? Does Kṛṣṇa represent hiṁsā or ahiṁsā? Did Kṛṣṇa classify the Kurukṣetra war as hiṁsā or ahiṁsā? ^§

^§ _{These days some people call the killing of bin Laden as ahiṁsā (to prevent him from killing from more people)}

Answer 1

"1a. Why didn't Kṛṣṇa really prevent the Mahābhārata war from taking place?"

The perception is wrong. Krishna & PAndava tried their best to prevent the war to happen.
As discussed in this answer, the premises of Mahabharata was completely one sided, i.e in favour of PAndava-s. The bottom line is that irrespective of Draupadi's insult & other treacheries, Kaurava agreed to return the kingdom after 12 + 1 years of exile. This is mentioned in SabhA parva:

_{Sakuni then said,--'The old king hath given ye back all your wealth. That is well. ... On the expiry of the thirteenth year, each is to have his kingdom surrendered by the other. O Yudhishthira, with this resolution, play with us}

This is a clear breach of treaty & trust.
If an employer doesn't pay 1 month salary, then workers go mad. Then just imagine about absolute humiliation in an assembly, worst insult of wife, followed by 13 years of brutal exile. Everyone has a threshold of "How much is too much".

"1b. Was the war really worth it?"

Despite all above, PAndava-s were still resilient. After the 13 years of exile, mostly they were so humbled, that PAndava-s had hardly any interest in attaining the kingdom as before they did.

Yudhishtira originally made the offer of just 5 villages (that's too for their own livelihood). I have covered PAndava-s view on war in this answer.
In those times, people were not "jack of all trades" like today. PAndava-s were good with rulership as being brought up in such family and that's what they could do to sustain. Some more quotes from Yudhishtira on his distress, if not given 5 villages:

_{...Kinsmen and friends and Brahmanas shun a poor man as birds avoid, O Krishna, a tree that beareth neither Rower nor fruits. Even this, O sire, is death to me that kinsmen shun me, as if I were a fallen one like the breath of life quitting 'a dead body. Samvara said that no condition of life could be more distressful than that in which one is always racked by the anxiety caused by the thought--I have no meat for today, what will become of me tomorrow? ... It is said that wealth is the highest virtue, and everything depends on wealth....}

The number of deaths in the war seems inflated. World war 2 is the greatest war with highest calamities. Even if it's otherwise, then also PAndava-s gave way more leniency to Duryodhana compared to Allied countries ignoring treaties broken by Hitler's Germany.

"1c. why not show the same viśva-rūpa to everyone in the battlefield and stop the war?

The last nail to coffin from Duryodhana was, attempting to capture Krishna who was a peace-ambassador. Jengiz Khan had ruined a country in middle east when his peace ambassadors were killed.
Krishna did show his universal form in the assembly as discussed in Udyoga Parva:

_{Krishna said: From delusion, O Suyodhana, thou regardest me to be alone, and it is for this, O thou of little understanding, that thou seekest to make me a captive after vanquishing me by violence. Here, however, are all the Pandavas and all the Vrishnis and Andhakas. Here are all the Adityas, the Rudras, and the Vasus, with all the great Rishis. Saying this Kesava, that slayer of hostile heroes burst out into a loud laughter. And as the high-souled Sauri laughed, from his body, that resembled a blazing fire, issued myriads of gods, each of lightning effulgence, and not bigger than the thumb. And on his forehead appeared Brahman, and on his breast Rudra. And on his arms appeared the regents of the world, and from his mouth issued Agni, the Adityas, the Sadhyas, the Vasus, the Aswins, the Marutas, with Indra, and the Viswedevas. And myriads of Yakshas, and the Gandharvas, and Rakshasas also, of the same measure and form, issued thence. And from his two arms issued Sankarshana and Dhananjaya. And Arjuna stood on his right, bow in hand, and Rama stood on his left, armed with the plough. And behind him stood Bhima, and Yudhishthira, and the two sons of Madri, and before him were all the Andhakas and the Vrishnis with Pradyumna and other chiefs bearing mighty weapons upraised. And on his diverse arms were seen the conch, the discus, the mace, the bow called Saranga, the plough, the javelin, the Nandaka, and every other weapon, all shining with effulgence, and upraised for striking. And from his eyes and nose and ears and every part of his body, issued fierce sparks of fire mixed with smoke. And from the pores of his body issued sparks of fire like unto the rays of the sun. And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, and Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya, and the Rishis, possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion.}

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"2. Gāndhārī asked Kṛṣṇa the same question (in the title); what was his response to her?"

The curse of GAndhAri & Krishna's initial response is discussed in this answer. Besides that,

_{The holy one said, ‘Arise, arise, O Gandhari, do not set thy heart on grief! Through thy fault, this vast carnage has taken place! Thy son Duryodhana was wicked-souled, envious, and exceedingly arrogant. Applauding his wicked acts, thou regardest them to be good. Exceedingly cruel, he was the embodiment of hostilities, and disobedient to the injunctions of the old. Why dost thou wish to ascribe thy own faults to me? Dead or lost, the person that grieves for what has already occurred, obtaineth more grief. By indulging in grief, one increases it two-fold.}

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"3. What exactly is Kṛṣṇa's view of hiṁsā or ahiṁsā? When to use hiṁsā and when to use ahiṁsā? Does Kṛṣṇa represent hiṁsā or ahiṁsā? Did Kṛṣṇa classify the Kurukṣetra war as hiṁsā or ahiṁsā?"

As discussed in this answer, "Violence" (Himsa) & "killing" are not 1:1 related. "Violence" is a kind of "violation" of Dharma, while "killing" is merely an act. A criminal being killed by police may not be a violence, however peaceful protesters being shot by police can be termed as violence. With that, the question of MahAbhArata being Himsa or Ahimsa becomes moot.
Krishna & many others consider it as a Dharma Yudh. If we consider Draupadi's understanding of Dharma as correct then, not killing a "killable", is also a sin.

O Janardana, that sin is incurred in slaying one that deserveth not to be slain. So there is equal sin in not slaying one that deserveth to be slain.

Violence is not related only to killing. As we seen above, killing can or cannot be violence. At the same time abusive language is considered as a violence, as discussed in this answer. The same was believed by Mahatma Gandhi as well, that violence can be done with action, speech and thoughts. In fact, due to twisted meaning of "non-violence", he also told following:

_{I do believe that, where there is only a choice between cowardice and violence, I would advise violence... I would rather have India resort to arms in order to defend her honour than that she should, in a cowardly manner, become or remain a helpless witness to her own dishonor.}

Krishna considers "Non-violence" as part of knowledge (gyAna).

_{BG 13.8 - Humility, un-pretentiousness, non-violence, for-bearance, sincerity, service of the teacher, cleanliness, steadiness, control of body and organs; ... is spoken of as knowledge}

Answer 2

https://www.sacred-texts.com/hin/m01/m01066.htm

"Vaisampayana said, 'Then Indra had a consultation with Narayana about the latter's descent on the earth from heaven with all the gods according to their respective parts. And, having commanded all the dwellers in heaven, Indra returned from the abode of Narayana. And the dwellers in heaven gradually became incarnate on earth for the destruction of the Asuras and for the welfare of the three worlds. And then, O tiger among kings, the celestials had their births, according as they pleased, in the races of Brahmarshis and royal sages. And they slew the Danavas, Rakshasas, Gandharvas and Snakes, other man-eaters, and many other creatures. And, O bull in the Bharata race, the Danavas, Rakshasas and Gandharvas and Snakes, could not slay the incarnate celestials even in their infancy, so strong they were.'

The Mahabharata war was a divine purpose to lighten the burden of earth and to slay asuras born in world of men for the welfare of the three worlds.

"And the earth, thus oppressed by the mighty Asuras endued with great strength and energy and possessed of abundant means, began to think of waiting on Brahman. The united strength of the creatures (such as Sesha, the Tortoise, and the huge Elephant), and of many Seshas too, became capable of supporting the earth with her mountains, burdened as she was with the weight of the Danavas.

This is why gods started to incarnate on earth.

"And, O bull of the Bharata race, when such was the blessed state of the terrestrial world, the Asuras, O lord of men, began to be born in kingly lines. And the sons of Diti (Daityas) being repeatedly defeated in war by the sons of Aditi (celestials) and deprived also of sovereignty and heaven, began to be incarnated on the earth. And, O king, the Asuras being possessed of great powers, and desirous of sovereignty began to be born on earth amongst various creatures, such as kine, horses, asses, camels, buffaloes, among creatures such as Rakshasas and others, and among elephants and deer. And, O protector of the earth, owing to those already born and to those that were being born, the earth became incapable of supporting herself. And amongst the sons of Diti and of Danu, cast out of heaven, some were born on the earth as kings of great pride and insolence. Possessed of great energy, they covered the earth in various shapes. Capable of oppressing all foes, they filled the earth having the ocean for its boundaries. And by their strength they began to oppress Brahmanas and Kshatriyas and Vaisyas and Sudras and all other creatures also. Terrifying and killing all creatures, they traversed the earth, O king, in bands of hundreds and thousands. Devoid of truth and virtue, proud of their strength, and intoxicated with (the wine of) insolence, they even insulted the great Rishis in their hermitages.

The asuras born on men started wreaking havoc on earth.

The earth was burdened by the weight of danavas.

Duryodhana and others such as Jarasandha were demons born on earth.

https://www.sacred-texts.com/hin/m01/m01068.htm

The first of Danavas, who was known by the name of Viprachitti, became that bull among men, noted as Jarasandha. And, O king, that son of Diti, who was known as Hiranyakasipu, was known in this world among men as the powerful Sisupala. He who had been known as Samhlada, the younger brother of Prahlada, became among men the famous Salya, that bull amongst Valhikas. The spirited Anuhlada who had been the youngest became noted in the world as Dhrishtaketu.

The evil-minded and wicked king Duryodhana, the destroyer of the fair fame of the Kurus, was born of a portion of Kali on earth. He it was who caused all creatures to be slain and the earth to be wasted; and he it was who fanned the flame of hostility that ultimately consumed all. They who had been the sons of Pulastya (the Rakshasas) were born on earth among men of Duryodhana's brothers, that century of wicked individuals commencing with Duhasasana as their first. And, O bull among the Bharata princes, Durmukha, Duhsaha, and others whose names I do not mention, who always supported Duryodhana (in all his schemes), were, indeed, the sons of Pulastya.

The Kauravas were portions of Danavas who repeatedly got defeated in war by Aditi's son and started to torture the earth by wreaking havoc and the Pandavas, Karna, Dhrishtadyumna and others such as Drupada, Virata, Kritavarma, Bhishma, Drona were portions of gods.

And, O king, know thou that Kama--the first of all exalted men--the foremost of all wielders of weapons--the slayer of foes--and the best portion of the maker of day--was the friend and counsellor of Duryodhana. And a portion of Sri herself became incarnate on earth, for the gratification of Narayana, in the line of Bhishmaka. And she was by name the chaste Rukmini. And the faultless Draupadi, slender-waisted like the wasp, was born of a portion of Sachi (the queen of the celestials), in the line of Drupada.

"And, O monarch, learn that king Yudhishthira was a portion of Dharma; that Bhimasena was of the deity of wind; that Arjuna was of Indra, the chief of the celestials; and that Nakula and Sahadeva, the handsomest beings among all creatures, and unrivalled for beauty on earth, were similarly portions of the twin Aswins.

Krishna was the portion of Narayana himself and Balarama was Shesha naga.

And he, called Vasudeva, endued with great valour, was among men a portion of him called Narayana--the god of gods--eternal. And Valadeva of exceeding strength was a portion of the Naga, Sesha.

And they all having resolved to come down on earth in their respected parts, then went to Narayana, the slayer of all foes, at Vaikunth--the one who has the discus and the mace in his hands, who is clad in purple, who is of great splendour, who hath the lotus on his navel, who is the slayer of the foes of the gods, who is of eyes looking down upon his wide chest (in yoga attitude), who is the lord of the Prajapati himself, the sovereign of all the gods, of mighty strength, who hath the mark of the auspicious whirl on his breast, who is the mover of every one's faculties and who is adored by all the gods. Him, Indra the most exalted of persons, addressed, saying, "Be incarnate." And Hari replied,--'Let it be.'"

https://www.sacred-texts.com/hin/m01/m01065.htm

Narayana was the one who said that the gods should incarnated on earth. So Vasudeva, born of his portion let the war to take place.

Vyasa also says that Krishna can change the very course of creation but he let the war to happen.

Krishna suffered it to take place although he was fully competent to baffle it. Govinda was able to alter the very course of the universe with all its mobile and immobile creatures. What need then be said of the curse of even high-souled Brahmanas? He who used to proceed in front of thy car, armed with discus and mace, through affection for thee, was the four-armed Vasudeva, that ancient rishi. That high-souled one of expansive eyes, Krishna, having lightened the burthen of the Earth and cast off his (human) body, has attained to his own high seat. By thee also, O foremost of men, with Bhima for thy helpmate and the twins, O mighty-armed hero, has the great work of the gods been accomplished. O foremost one of Kuru’s race, I regard thee and thy brothers as crowned with success, for ye have accomplished the great purpose of your lives.