Thirteen Day War
From Gineipaedia, the Legend of Galactic Heroes wiki
The Thirteen Day War (Japanese: 十三日戦争) was a nuclear conflict fought on Earth in 2039 CE between the United States of Eurafrica and the Northern Condominium. Both factions used nuclear weapons against each other and suffered from mutual destruction in less than two weeks. By the end of the brief but catastrophic war, the vast majority of Earth's major cities were destroyed and the human population had been reduced to a mere one billion. (LOGH: 'To Earth')
The war would end without a victor as both sides had reduced each other to ashes. With nearly all of their infrastructure destroyed, the survivors, and what was left of their governments, quickly descended into chaos, which was eventually ended some 90 years later with the formation of the United Earth Government in 2129 CE. One of the few cities to survive the Thirteen Day War, Brisbane, Australia, was selected to be the UEG capital.
The unfathomable destruction caused by the use of nuclear weapons greatly added to their stigma, and over a millenium later their use directly against an inhabited planet was still considered to be taboo.