This began a long stalemate period between the Earth Federation and the Principality of Zeon, with the Earth Federation slowly giving ground until the Zeon had control over approximately two thirds of Earth's surface. With the Earth Federation Space Forces devastated, Zeon pressed its advantage by staging an invasion of Earth, dropping mobile suit forces onto the planet during several operations in March of UC 0079. After this, both parties agreed to sign the Antarctic Treaty which, among other things, forbade the use of weapons of mass destruction. During this period of fighting, each side lost nearly half of their respective populations.

During the fierce fighting in the first month of the war, weapons of mass destruction were used indiscriminately, with Zeon forces deploying G3 Gas in inhabited colonies, attempting a colony drop against the EFF's headquarters at Jaburo during Operation British, and the Principality of Zeon using nuclear weapons during the Battle of Loum. It began on January 3, UC 0079, when the Principality of Zeon launched surprise attacks against the Earth Federation controlled Sides. The One Year War was the single largest and most devastating conflict of the Universal Century, in both the number of military casualties, civilian deaths, and devastation across the entire Earth Sphere. In January UC 0079, the Principality of Zeon officially declared war on the Earth Federation. Zeon entered a state of national mobilization, dividing their military into a Space Attack Force under the command of Dozle Zabi and a Mobile Assault Force under the command of Kycilia Zabi.

Declaring themselves the Principality of Zeon a year later, and brutally purging Zeon Deikun's remaining followers, Side 3 became increasingly hostile to the Earth Federation. Relations between the two had been strained since Side 3 declared themselves the independent Republic of Zeon under the leadership of Zeon Zum Deikun, and had only worsened since Deikun's death and the Zabi family's rise to power. The One Year War was the culmination of the hostilities between Side 3's Principality of Zeon and the Earth Federation.

Earth federation mobile suits