Why was the war fought?

Over a throne, and over a humiliation that made compromise impossible. Dhritarashtra, the elder brother, was born blind and so his younger brother Pandu ruled; when Pandu died, Dhritarashtra held the throne and his eldest son Duryodhana claimed it as his own, while Yudhishthira, Pandu's eldest, claimed it as the son of the king who had actually reigned. The succession was arguable from the start.

The kingdom was divided, the Pandavas built Indraprastha, and Duryodhana invited Yudhishthira to a game of dice played with Shakuni's loaded dice. Yudhishthira lost his wealth, his brothers, himself and finally Draupadi, who was dragged into the assembly hall and stripped while the elders said nothing. The settlement was thirteen years of exile, the last year in disguise, after which the kingdom was to be returned. It was not returned.

Krishna went to Hastinapura as the Pandavas' envoy and asked for five villages instead of a kingdom. Duryodhana refused to give land that could be covered by the point of a needle. After that, both sides spent months recruiting allies across the subcontinent.

How big were the armies?

The epic measures armies in akshauhinis. One akshauhini is given a precise composition: 21,870 chariots, 21,870 elephants, 65,610 horse and 109,350 foot, a total of 218,700 men and animals. Eighteen akshauhinis gathered at Kurukshetra, eleven for Duryodhana and seven for Yudhishthira, which the epic's own arithmetic puts at close to four million combatants.

These are literary numbers, and the number eighteen governs the whole poem: eighteen books, eighteen akshauhinis, eighteen days, eighteen chapters in the Bhagavad Gita. Before the fighting the two sides agreed on rules of war - no attacking the unarmed, no fighting after sunset, one combatant against one, no striking below the navel with a mace. Nearly every rule is broken before the end, and the breaking is the point.

Who commanded each side?

The Pandava army had one commander-in-chief for the whole war: Dhrishtadyumna, son of Drupada and brother of Draupadi, born from a fire altar for the express purpose of killing Drona. Shikhandi, Virata, Drupada, Satyaki and Bhima led divisions under him, and Krishna drove Arjuna's chariot without bearing arms.

The Kaurava command changed four times as each holder fell. Bhishma led for the first ten days, Drona for days eleven to fifteen, Karna for days sixteen and seventeen, and Shalya for the morning of the eighteenth. That night, with Duryodhana dying, Ashwatthama was named commander of an army that no longer existed.

What happened, day by day?

Day one opens with Arjuna refusing to fight and Krishna answering him: the Bhagavad Gita occupies the pause between the armies. The fighting then goes badly for the Pandavas, who lose Virata's son Uttara and his brother Shveta almost at once.

The first ten days belong to Bhishma, who cannot be killed against his will and who slaughters the Pandava ranks while refusing to kill any of the five brothers. On the tenth day the Pandavas put Shikhandi, born female and now a man, in front of Arjuna's chariot. Bhishma lowers his bow rather than fight him, and Arjuna shoots him down. Bhishma falls on a bed of arrows and does not die; he waits on the field for the winter solstice, teaching, through the next two books.

Drona takes command and the war turns cruel. On the thirteenth day he forms the chakravyuha, a spiral battle array, and the sixteen-year-old Abhimanyu breaks in alone because he knows how to enter and not how to leave. Six great warriors surround him and kill him, breaking the rules of single combat. Arjuna swears to kill Jayadratha, who blocked the other Pandavas at the mouth of the formation, before sunset the next day or burn himself alive; on the fourteenth day, with the sun apparently set, Jayadratha shows himself and Arjuna beheads him. The fighting runs on into the night, and Karna finally uses the one-use divine spear he had saved for Arjuna to kill Bhima's son Ghatotkacha, who was destroying the Kaurava army in the dark.

On the fifteenth day Yudhishthira is persuaded to announce that Ashwatthama is dead - meaning an elephant of that name - and Drona, hearing it, lays down his weapons and is beheaded by Dhrishtadyumna. Karna leads for two days and falls on the seventeenth, shot by Arjuna while his chariot wheel is stuck in the ground and he is unarmed, after Shalya has spent the day insulting him. On the eighteenth Shalya dies, Shakuni is killed by Sahadeva, and Bhima smashes Duryodhana's thigh with a mace in a duel by the lake at Samantapanchaka - a foul blow, made at Krishna's prompting.

What was the night raid?

The eighteenth book of fighting is not a battle at all. With Duryodhana dying, Ashwatthama, Kripa and Kritavarma enter the sleeping Pandava camp after dark. Ashwatthama kills Dhrishtadyumna, Shikhandi and the five young sons of Draupadi in their beds, and the other two cut down anyone who runs. The five Pandavas and Krishna survive only because they are not in the camp that night.

Pursued the next morning, Ashwatthama looses a brahmashira weapon at the unborn child in Uttaraa's womb, the last of the line. Krishna revives the child, who is born as Parikshit, and curses Ashwatthama to wander for three thousand years. The war that began over who should inherit ends with almost nothing left to inherit.

Who survived, and what came after?

Ten men. On the Pandava side: the five brothers, Krishna, Satyaki, and Yuyutsu, Dhritarashtra's son by a vaishya woman, who crossed over to the Pandavas before the first day. On the Kaurava side: Ashwatthama, Kripa and Kritavarma.

The books that follow the fighting are about the cost. The women come out to the field to find their dead and Gandhari curses Krishna for letting it happen. Yudhishthira wants to abdicate and is talked out of it over thousands of verses of instruction from the dying Bhishma. Kunti finally tells her sons that Karna, whom they killed, was their elder brother. Thirty-six years later Krishna's own clan destroys itself, and the Pandavas walk into the Himalaya to die.

Key facts

Length: 18 days
Place: Kurukshetra, in modern Haryana
Armies: 18 akshauhinis - 11 Kaurava, 7 Pandava
One akshauhini: 21,870 chariots, 21,870 elephants, 65,610 horse, 109,350 foot
Kaurava commanders: Bhishma 10 days, Drona 5, Karna 2, Shalya 1
Pandava commander: Dhrishtadyumna, throughout
Survivors: Ten, including Krishna

Sources

Mahabharata, Bhishma, Drona, Karna, Shalya and Sauptika parvas, in the translation of K. M. Ganguli (1883-1896).
Bibek Debroy, The Mahabharata, vols. 5-8 (Penguin, 2012-2013).
Alf Hiltebeitel, The Ritual of Battle: Krishna in the Mahabharata (1976).

Where the guides summarise a disputed point, they say so. How this site is put together.