Declaring themselves the Principality of Zeon a year later, and brutally purging Zeon Deikun's remaining followers, the Principality became increasingly hostile to the Earth Federation. Relations between the two had been tense since Side 3 declared themselves the independent Republic of Zeon under the leadership of Zeon Zum Deikun in UC 0058, and had only worsened since Deikun's death and the Zabi family's rise to power in UC 0068.

The One Year War was a culmination of the hostility between Side 3's Principality of Zeon and the Earth Federation. The war ended on January 1, UC 0080, when the Republic of Zeon, reinstated after the defeat and isolation of the Principality, signed a peace treaty with the Earth Federation. The portion of the war depicted in the original anime, Mobile Suit Gundam depicts mainly the last few months of the 'One Year War', where only the introductory narration mentioned the earlier events including destruction of multiple space colonies and the dropping of an Island 3 type colony from orbit, resulting in the destruction of half of the human population. A short period in the first week of the war is known as the 'One Week Battle' or the 'One Week War', in which most civilian casualties are accrued by the indiscriminate use of WMDs and surprise attacks. The One Year War begins on January 3, UC 0079 (the Universal Century calendar begins some time after 2045 AD, according to the official timeline).

The war is also known as the Zeon's War of Independence to the former soldiers of the Principality of Zeon. Its casualties, both military personnel and civilians were in the billions. The One Year War is the largest military conflict depicted in the Universal Century series, involving tens of thousand of mobile weapons, thousands of war ships and millions of combatants.

Brought about the beginning of Mobile Suit Warfare.

Heirshy takes over the Party until UC 0084.

Humanity Recovers from It's half extinction.

Principality of Zeon under authority of the Treaty of Grenada.

January 3rd, UC 0079 - January 1st, UC 0080

Earth federation mobile suits