After Deikun's death, his advisor, Degwin Sodo Zabi, became the new leader and renamed the Republic the Principality of Zeon, changing its flag as well. Degwin rapidly expanded the colony's national guard units into a full fledged army. Mobile suits and other unique weapons made their first appearance. War was soon declared unilaterally on the Earth Federation. Outside of the armed forces ranks, Degwin Sodo Zabi is sometimes referred to as the "Sovereign of Zeon".
The resulting conflict against the Federation was known as the One Year War. The first week of the war, known as the "One Week Battle" (from January 3 U.C. 0079 to January 10 U.C. 0079) resulted in the deaths of 3 billion people (2.8 billion colonists, by indiscriminate use of NBC weapons, and an estimated 200 million from nuclear winter, as a result of Operation British) and the total destruction of Side 1, 2 and 4. Following the Battle of Loum on January 15 U.C. 0079 killed most residents of the Side 5 colony. The combined attacks during this period resulted in the death of nearly half of the total human population around the Earth sphere.
Initial estimations put the death toll of the colony drop at 2.32 billion: 0.32 billion as a direct result of the drop itself, and an additional 2 billion due to the drop's after-effects (that is, many places in the vicinity of the drop zone losing contact with the rest of the world, and thus losing any aid in their survival). Post-war research, however, lowered the number to a total of 0.2 billion deaths directly related to the drop. As a consequence of the astounding losses, the Earth Federation and the Principality of Zeon signed the Antarctic Treaty on January 31 U.C. 0079, banning the use of NBC weapons and colony drops.
The next day, immediately after the treaty was signed, Zeon began the formation of its Zeon Earth Attack Force. By the middle of U.C. 0079, the Zeon forces had controlled over 70% of the Earth's surface. Nevertheless, after several major operations (Velvet, Odessa, et al.), the Earth Federation regained much of their lost ground. The war eventually climaxed in the Battles of Solomon and A Baoa Qu, where the Earth Federation's superior forces took control of both Zeon fortresses. The Principality of Zeon finally surrendered with the Treaty of Granada, on January 14, U.C. 0080.
Three years later, in U.C. 0083, the Delaz fleet, under Aiguille Delaz, sought to rebuild Zeon in Operation Stardust. The resulting colony drop destroyed much of the North American farmland. The Delaz fleet was soon defeated, but the war led to the Earth Federation's creation of a force called the Titans to prevent future insurrections.
Read more about this topic: Zeon