The Krypton-Shi'ar War was an intergalactic conflict that took place between 1970 and 1976. The war was waged between the people of Krypton and the Shi'ar Empire. The war ended with the Shi'ar forced to mediate peace with Krypton.
Krypton had regressed to a peaceful and near-isolationist state after their conflict with the Skrulls and Kree for thousand of years. Peacetime reigned until the Shi'ar Emperor Kral'Nor led an expansive policy that threatened the Xerius Galaxy where Krypton is located. Originally, the Kryptonians had maintained a cordially inter-planetary relationship prior to Kral'Nor's rise to power. Kral'Nor had vested interest in Krypton's World-Gate network that was recently reactivated and demanded in utilizing it to conquer nearby star systems. The Kryptonians tried to meditate with Kral'Nor until they had no choice but to call back every Kryptonian to Krypton, and shut down the network. As a result, Kral'Nor effectively declared war on Krypton.
Due to the Kryptonians becoming more restricted to their home planet, the planetary government's Council of Twelve maintained a defense policy as their military forces fall back to Krypton and constructed a Planet-Shield around the planet. Vastly outnumbered, Krypton was besieged by the Shi'ar for six years. During this period, General Dru-Zod, the head of the Kryptonian Defense Force, was frustrated by the stagnancy of the Council that was becoming apparent and their unwillingness to strike back at the Shi'ar, and plotted a coup in an effort to remove the Council. However, the coup did not came through as Zod's lover Ursa-Ka was killed in combat; in which Ursa's death drove Zod to call off the coup and resume a vendetta on the Shi'ar.
On the sixth year of the siege, General Zod and scientist Jor-El led a assault against the Shi'ar; in which they utilized the World-Gate network by opening one World-Gate and allowing a small team, including Jor-El and another scientist, Yen-Lir, led by Zod broke through the Shi'ar lines, and activated the Gate heading to Krypton's most powerful arsenal, Battlestation Sentrius. They managed to move Sentrius into position next to the Gate where Zod used the Gate as a tactical weapon; he fired Sentrius' weapons through the open Gate, programming them to automatically lock on to Shi'ar ships, which then completely wiped out the Shi'ar fleet. The heavy losses was a humiliating blow to the Shi'ar that led to Emperor Kral'Nor being forced to mediate peace with Krypton.
Kral'Nor and his family enacted the peace terms at Krypton's capital city Kryptonopolis, where it was attended by General Zod and Jor-El. Under the peace terms, the Shi'ar Empire is to withdraw from the Xerius Galaxy, dismantle of any and all Shi'ar warship-construction and weapons-production facilities constructed, and submit to yearly inspection of military strength by the Green Lantern Corps over a period of ten years. And if failure to do so would forced Krypton to allow Sentrius to remove them. During the peace signing, Kral'Nor's son Prince D'Ken was enraged by the humiliations that were being addressed by Zod and sworn that he will have revenge on Krypton and its people.
The war had taken a heavy toll on the Shi'ar combined with the number of casualties and a major economic depression. As a result, Kral'Nor was overthrown by a group of Shi'ar that opposed the war and replaced by his youngest daughter Lilandra Neramani. Under Lilandra, the Shi'ar Empire was dissolved and reorganized as the Shi'ar Republic. Kral'Nor was banished along with his family and the forces that were loyal to him.
It is during the end of the war, General Zod authorized his own accord in wiping out a Shi'ar fleet with a Eradicator and reasoned his actions as a show of force to the Shi'ar from ever attacking Krypton. Zod was subsequently placed on trial by the Council for war crimes and was banished into the Phantom Zone.
But shortly in the aftermath of the war, Krytpon's longtime and forgotten nemesis, the Dheronians, took advantage of the Kryptonians' time of peace and teleported a series of probes into Krypton's core and causing its subsequent destruction.