Book 7: Drona
With Bhisma vanquished, Duryodhana appoints Drona as his commander. Drona is in his 80s, but fights like he is 16. He is also the man who taught Arjuna how to fight. Duryodhana demands that Drona kidnap Yudhisthira and bring him to Duryodhana. Killing Yudhisthira would infuriate Arjuna and cause him to avenge his brother's death, but if Yudhisthira is kidnapped then Duryodhana can defeat him in another game of dice. Drona vows to caputre Yudhisthira, but this will mean distracting Arjuna so that Yudhisthira is left unprotected.
After attempting this capture and failing, Drona then promises Yudhisthira that he will kill one of the key Pandavas. This results in a campaign where Drona and Karna team up on Abhimanyu, Arjuna's son. After a trying battle between Karna and Abhimanyu, Arjuna's son is ultimately killed, sending shockwaves across the battlefield. When Arjuna learns of his son's death, he vows to kill Jayadratha, who had attempted to molest Draupadi during the Pandavas' time in exile. Drona resumes his attempts to kidnap Yudhisthira, but is now tasked with protecting Jayadratha as well.
When Arjuna and Drona meet in battle, Arjuna backs away in the name of pursuing Karna instead. Drona taunts Arjuna, asking why he would leave without killing his enemy, and Arjuna tells Drona that he is not an enemy, but his teacher. After a series of Pandava losses on the battlefield, Bhima goes to fight Drona himself and lays waste to Drona's armies. In the midst of the chaos, Yudhisthira is left unprotected, making him vulnerable to being captured. When Arjuna learns of this, he finds himself in quite the bind. He must protect Yudhisthira and kill Jayadratha, yet the sun is setting.
Arjuna beheads Jayadratha with an arrow and parades around the arrow with Jayadratha's severed head attached. As retribution, Karna kills Ghatotkaca with a weapon he originally intended to use on Arjuna, but decides against because it would invite too great retribution. The Pandavas then devise a plan to kill Drona: Bhima tells him that he has killed Asvatthaman. Asvatthaman is the name of Drona's son, but Bhima is playing a sly trick, referring to an elephant that he just killed on the battlefield. In a fit of rage, Drona violates his dharma and kills tens of thousands of soldiers. When confronted with his violation of dharma, Drona lays down his arms and prays that Karna stays protected. Drona accepts his death at the hands of Dhrstadyumma, and the Pandavas celebrate a massive victory.
Book 8: Karna
Upon learning of Drona's death, Duryodhana finally grasps the gravity of his errors for provoking the Pandava brothers. He appoints Karna, who is resolved to kill Arjuna. Salya tries to dissuade Karna from going into battle with Arjuna since Arjuna and Krsna are invincible and have more powerful weapons, but Karna shows his bitterness when he recounts the time when he himself sought the cosmic weapons only to see them given to Arjuna instead. Karna swears to kill Arjuna with his own weapons. After a short period of fighting, Yudhisthira and Arjuna get in a spat over the fact that Arjuna has yet to kill Karna, which results in Arjuna having to be talked out of Yudhisthira, Yudhisthira suffering Arjuna's rudeness and vowing to abdicate his kingdom to Bhima, and the two ultimately resolving their issues to join forces again.
When Arjuna does finally meet Karna on the battlefield, the fight is protracted and bloody. Arjuna systemically brutalizes Karna by destroying his armor and eventually his body. Whenever Karna fires arrows at Arjuna, Arjuna cuts those arrows. In response to Karna using his own most powerful weapons, Arjuna unleashes the Weapons of Indra, and these end up killing Karna. There's a chilling image after Karna's death of his spirit descending from his body.
Book 9: Salya
Duryodhana is devastated by the death of Karna and begins to realize that his efforts are doomed. On the recommendation of his confidants, he appoints Salya, the Madra king and the new commander of the army, to continue the fighting. Early on, everyone is evenly matched, with Asvatthaman fighting Arjuna as an equal. This changes when Yudhisthira becomes infuriated that Salya is the new commander, and he vows to kill the king personally himself. Yudhisthira meets Salya in the battlefield and shows a new rage, which leads him to defeat Salya handily with arrows.
Following Salya's death, Duryodhana instructs his armies to continue fighting, either for victory or for a glorious death. Packs of Duryodhana's soldiers then begin attacking the Pandavas without Duryodhana's express permission. When the Pandavas go looking for Duryodhana to kill him, they can not find him and retreat for the night. But soon enough, Bhima learns that Duryodhana is hiding in a lake, and the brothers go there and challenge him. The grounds of the challenge are that if Duryodhana can kill one of the Pandavas with a weapon of his choosing, he will get the kingdom back. He agrees, and chooses a club. Bhima volunteers to fight first, but Krsna warns him that he may lose, as Duryodhana may outsmart him. Bhima chooses to fight anyway.
Balarama suddenly returns from a pilgrimage—and the story is briefly interrupted to tell of his pilgrimage encountering various gods—and suggests that the fight is staged at Samantapanacka, since any warrior who dies there is guaranteed to go to heaven. The battle ends quickly, after Bhima strikes Duryodhana below the navel, killing him. Balarama is infuriated, since this is a forbidden form of fighting, and claims Bhima violated dharma. Duryodhana, while dying, protests Bhima's strike as well, but Krsna gives the final ruling on dharma, saying that Duryodhana had acted in adharma since the beginning, and that the Pandavas were acting in dharma by fighting him in a war and defeating him. While Duryodhana ultimately accepts his death, Asvatthaman is infuriated by the way both his father and Duryodhana were killed, and desires retribution.
V.S. Sukthankar, one of the preeminent scholars of the Mahabharata, explores the rich and sometimes contradictory nature of how this war is depicted in his book On the Meaning of the Mahabharata. He states that, like with Homer's Iliad, the tragedy of the Mahabharata is a massively destructive war that is, at the end of the day, an excessive act born out of a provocation that was incommensurate with the cost. In both epics, the world is effectively destroyed by a war that should not have transpired in the first place. The war at the center of the Mahabharata was not inevitable but, instead, entirely avoidable.
It's these type of essential contradictions that drive the story in the epic and also set up its most potent and pertinent character interactions. The battle between Arjuna and Drona is not positioned as that between the righteous and the sinner like the battle between their respective kings Yudhisthira and Duryodhana is, but rather as the meeting on the battlefield between two fearsome warriors who share a deep kinship. As Drona is Arjuna's former mentor, there is a mutual respect between them. Dharma pervades this teacher–pupil relationship as they now must fulfill their individual dharmas as warriors by confronting each other on behalf of the forces they represent.
Drona is portrayed as a somewhat paradoxical character. He is an 85-year-old that fights like he's 16, and he is the commander of an army fighting on behalf of the wicked Duryodhana yet himself commands respect. As Sukthankar notes, Drona is portrayed as wearing white, the "color of purity and equanimity." How can Drona himself be so pure, so righteous, if he fights on behalf of a force of evil? Here, we should leave aside any Judeo-Christian notions of good and evil. Drona is not beholden to fight for the righteous, but to abide by his own dharma and fulfill his fate.
Drona does not violate his dharma by fighting on behalf of Duryodhana, but by needlessly slaying. He lays down his arms to accept his fate, as giving himself over to his rightful death is the virtuous move. The idea that a soldier's death on the battlefield is a virtuous one recurs with the deaths of all of Duryodhana's commanders: be it with the spectacular image of Karna's soul ascending from his body or with the negative example of Duryodhana hiding in a lake to escape the impending death that is rightfully his. In all of these cases, death is not portrayed as something accidental or incidental, but as an inevitable event preordained by some greater force. For a warrior, accepting that death is acting with detachment in the name of dharma.