The Pandavas were merely a pretext. Krishna used the Pandavas to rid the world of evil men. You have already learnt that one of Yayati’s sons was Puru. In Puru’s line were born Bharataa and Kuru. One of Kuru’s descendants was the king Shantanu. Shantanu married Ganga and Bhishma was born from this marriage.
But Shantanu also married Satyavati and had two more sons, Chitrangada and Vichitravirya. Bhishma never married. Chitrangada died young. When Vichitravirya grew up, Bhishma defeated the king of Kashi and brought two of the king’s daughters, Ambika and Ambalika, as brides for Vichitravirya. Vichitravirya as also quite young when he died of tuberculosis.
Since Vichitravirya had left no children, Vyasadeva was brought to Hastinapura. Vyasadeva and Ambalika had a son named Dhritarashtra and Vyasadeva and Ambalika had a son named Pandu. Dhritarashtra married Gandhari and they had a hundred sons, of whom the most important was Duryodhana. Pandu had two wives, Kunti and Madri. Kunti’s sons were Yudhisthira, Bhima and Arjuna and Madri’s sons were Nakula and Sahadeva. But Yudhisthira was really the son of the god Dharma and not Pandu’s son. Similarly, Bhima was the son of the god Pavana, Arjuna the son of Indra and Nakula and Sahadeva the sons of the two Ashvinis. Earlier, Kunti had a son named Karna from the sun-god.
This was before she had god married to Pandu. Karna became a friend of Duryodhana’s. Because of a curse imposed on him by a sage, Pandu died in the forest.
Duryodhana tried his best to kill the Pandavas. He set fire to a house of lac (jatugriha) in which Kunti and the five Pandavas were staying. But the Pandavas were saved and fled to a city named Ekachakra. There they lived, disguised as brahmanas. In Ekachakra, they destroyed a rakshasa named Vaka. They then won the hand of the daughter of the king of Panchala. Her name was Droupadi and all five Pandava brothers married her. When Duryodhana learnt that the Pandavas were alive, he handed over half the kingdom to them.
Meanwhile, the forest Khandava had to be burnt and Krishna and Arjuna did this together. Krishna had befriended Arjuna. When Arjuna successfully defeated the god Agni at the burning of the Khandava forest, Agni gave him several divine weapons. Arjuna had also obtained divine weapons from his guru Dronacharya.
On the Pandava side, Yudhisthira had become king. The Pandavas organised a rajasuya yajna (royal sacrifice) in which they conquered several kingdoms and accumulated lot of wealth. This made Duryodhana envious.
He arranged a game of dice (aksha) between Yudhisthira and Duryodhana’s uncle Shakuni. Shakuni did not play fairly and Yudhisthira lost the game. As penalty for the loss, the Pandavas were to spend twelve years in the forest and one additional year without being detected. Droupadi went with them to the forest, as did the Pandava’s priest, Dhoumya.
After the twelve years were over, the Pandavas came to the kingdom of King Virata where they proposed to spend the additional year that had to be spent in disguises. Yudhisthira pretended to be a brahmana, Bhima a cook, Arjuna a dancer, Nakula and Sahadeva stable-hands. Droupadi became the queen’s maid. The queen’s brother Kichaka tried to molest Droupadi, but was killed by Bhima. When the year was over, the Kauravas attacked King Virata to rob him of his cattle. But Arjuna defeated all the Kauravas and saved Virata’s cattle. After this success, the identity of the Pandavas could no longer be kept a secret. But thankfully, the one year during which identities had to be kept a secret, was over.
King Virata’s daughter Uttara was married to Abhimanyu, Arjuna’s son. Abhimanyu’s mother was Subhadra, whom Arjuna had married. Subhadra also happened to be Krishna’s sister.
The Pandavas now demanded their rightful share of the kingdom, but Duryodhana refused. A war was imminent. A huge battalion of soldiers was known as an akshouhini. Duryodhana collected eleven akshouhinis for the war and Yudhisthira collected seven. Krishna was sent as a messenger to Duryodhana to try and preserve the peace. Krishna told Duryodhana that the Pandavas would be satisfied with a mere five villages. Duryodhana refused to give them even this without a fight.
So the armies gathered for a war on the plains of Kurukshetra. Noticing that elders and relatives like Bhishma and Dronacharya were fighting on the side of the Kaurvas, Arjuna was reluctant to fight. But Krishna gave Arjuna lessons which have come down to us as the Gita. He taught there was no reason for sorrow if Bhishma or Dronacharya died, that was only a death of their physical bodies. The true identity of a person was his atman (soul) which never died, but passed from one body to another. True bliss was obtained when the atman united with the Brahman (divine essence) or Paramatman (supreme soul). This was always the goal of a yogi, that is, a person who sought union with God.
Thus instructed by Krishna, Arjuna started to fight. With the help of Shikhandi, he defeated Bhishma. This happened on the tenth day of the fighting. Bhishma did not however die. He had earlier received the boon that he would only die when he actually wished to do so. For many days, he lay there in the battlefield on a bed of arrows. After Bhishma’s defeat, Dronacharya became the general on the Pandava side. Dronaharya killed Virata, Drupada and several other kings and soldiers on the Pandava side. Dhrishtadyumna also killed many Kaurava soldiers. On the fifteenth day of the fighting, a rumour gained currency that Ashvatthama, Dronacharya’s son, had been killed. Dronacharya abandoned his weapons on hearing this bad news and Dhrishtadyumna faced no problems in killing him. Karna now became the Kaurava general and lasted for two and a half days before he was killed by Arjuna. Shalya was the last Kaurava general. He fought for only half a day and was killed by Yudhisthira.
Bhima and Duryodhana fought the last duel of the war with maces. Bhima broke Duryodhana’s thighs and killed him. Ashvatthama had been fuming ever since his father Dronacharya had been killed by unfair means. In the dead of the night, he entered the Pandava camp where he killed Dhrishtadyumna and the five sons of Droupadi. Droupadi was disconsolate and demanded revenge. Arjuna and Ashvatthama let loose divine weapons at each other. Since this might destroy the world, they were asked to withdraw these weapons. Arjuna could withdraw his weapon, but Ashvatthama could not. Ashvatthama’s weapon killed the baby that was in Uttara’s womb, but when the dead baby was born, Krishna brought it back to life. This baby was Parikshita.
Many kings and soldiers died in the course of the Kurukshetra war. The only ones left alive were Kritvarma, Kripacharya and Ashvatthama on the Kaurava side and Pandava side. After the war was over, Bhishma taught Yudhisthira the duties of king. It was only after this that he died.
As a king, Yudhisthira performed many yajnas and gave a lot of to brahmanas. When Yudhisthira learnt that the Yadvas had been destroyed, he no longer wished to rule. He handed over the kingdom to Parikshita and the Pandavas left on a pilgrimage, in the course of which they died.
It was Krishna who had used the Pandavas as a tool to rid the world of evil kings and establish the good ones. Realising that the Yadavas were also evil, Krishna also ensured that the Yadavas would be destroyed. He then gave up his life at the place of pilgrimage that is known as Prabhasa. After Krishna died, the city of Dvarka was swallowed up by the sea. This was the story of the eighth avatara of Vishnu.