The Battle of Krypton was a coup orchestrated by General Zod in an attempt to take control of Krypton from the Council of Five. It was Zod's attempt to correct Krypton's leadership in hopes of saving his planet from further destabilization and, ultimately, destruction, but the actions of the Sapphire Guard neutralized the Sword of Rao after trapping Zod, ending the war. In the aftermath of the battle, Zod and the Sword of Rao were sentenced to the Phantom Zone.
During the Kryptonian Expansion, artificial population control was implemented on Krypton, leading to the space exploration program being abandoned as genetic cultivation was further explored, exhausting Krypton's natural resources. In response, the Council of Five authorized the mining of Krypton's core.
Zod, leader of the Warrior Guild, formed the Sword of Rao, and joined by his subordinates. His influence within the Warrior Guild also led many soldiers to join in, although many didn't join in, being loyal to the Council which were democratically elected.
Jor-El went a different way: knowing that all population in Krypton were genetically bred for a role in society, he knew that any change to the order would spell massive chaos, thus going to the conclusion that everyone on the planet is dead already, way before the planet's implosion. He and his wife, Lara Lor-Van decided to gain access to the Growth Codex, the registry that contains the genetical information of every individual in Krypton; and by trying to bear a child for the first time in Kryptonian history after the implementation of the population control. Jor-El's plan was to use the codex and infuse it to his child, and sending the child away to a planet with a yellow star in Sector 2814 (Earth), and potentially going there when it is possible. The first part was successful, Lara and Jor-El managed to conceive a child: Kal-El. However, at this time, he wasn't able to gain access to the codex, as it is a criminal offense to do so, thus he went to the council in an attempt to do so.
Coup and War
Jor-El went to the Council with his warning and plan. The Council, skeptical of this decided to deliberate again, much to Jor-El's dismay. At the same time, Zod and his followers launched the coup to save Krypton. Zod went and announced that the Council is disbanded, killing High Eminence Ro-Zar and arresting the other Council members. Seeing Jor-El, Zod was excited as his old friend had agreed that the planet was dying, however Jor-El did not agree with Zod's plan for keeping the bloodlines, thus disappointing Zod. Still considering Jor-El a friend, he decided to take Jor-El away while he moves to other matters, namely to consolidate his influence as a new leader.
Outside, his forces clashed with Loyalist forces, seemingly on equal standing. On Kandor, chaos ran free as capital ships fought on the skies, and assault ships ruins Kandor. Jor-El escaped captive and saw the chaos and decided to act on his own. Grabbing a steed, he went to a Genesis Chamber where the Codex was located and stole it to infuse the genetic information in his son. Zod knew of this and sends 2 attack ships to intercept him, although Jor-El managed to escape back to his citadel.
On Kandor, Zod's forces are being defeated as reinforcements came for the Loyalists from other parts of Krypton. On this, Zod lead a squadron to Jor-El's citadel to arrest him. Meanwhile, Jor-El managed to infuse the codex to his son and prepared a ship to send him away, while also including a command key for use of a Kryptonian scout ship on Earth. Informed by Kelex, his service droid, Jor-El readies his Battle Armour to face Zod. In the citadel, Zod knew what had been done, and in shock found out about a natural birth, which is considered heresy. Finally losing patience with his old friend, Zod attacked Jor-El in combat. Zod, being the leader of the Warrior Guild, is considered a better fighter, however Jor-El grew up alongside him and learned also to fight, and in this fight, Jor-El managed to hold off his own and defeat Zod in hand-to-hand combat. Defeated, Zod plead to Lara to cancel the ship's departure but failed, and the ship began its escape from Krypton; and Zod, enraged, used his blade gauntlet to kill Jor-El, went out and ordered a ship to destroy Kal-El. In the last minute, a Loyalist capital ship destroyed Zod's ship and major reinforcements from the Loyalists managed to end Zod's coup attempt and defeat the Sword of Rao, Zod's insurrection had failed.
After the coup attempt, the Law Council regained control over Kandor and Krypton; while also sentencing the crimes committed by the Sword of Rao. As it is the most heinous and high-profile crime done in Krypton for centuries, the Council sees it right to put the war criminals through 300 years of somatic reconditioning in the Phantom Zone. Before being sent to the Phantom Zone, Zod declared to Lara that he will find Kal-El, no matter where he is sent. Afterwards, all of the surviving war criminals were put to cryogenic capsules and sent on the Black Zero to the Phantom Zone.
Not long after, Jor-El's prediction came true; Krypton imploded. Lara could only agree in vain with Jor-El and hoped that Kal-El would avoid the same mistakes Krypton made as Krypton's core gave way and imploded the whole planet, killing all life. As the planet imploded, the ripple effect of its destruction broke the boundaries of the Phantom Zone and freeing Zod and the Sword of Rao in the Black Zero. Distraught at the remains of their dead homeworld, Zod and his allies reconfigured the Black Zero into a starship and began searching ancient Kryptonian outposts for life, but found nothing but death. During their 33-year search, they scavenged armor, weapons, and a World Engine from which they might rebuild Krypton, until one day, they received a distress beacon from an ancient Kryptonian scout ship activated by Clark Kent which led them to Earth.