The Universal Century is the Gundam timeline seen in the original Mobile Suit Gundam anime and its various spinoffs, sequels and prequels. In that timeline, one single event dominates the Universal Century and sends ripples throughout the era, setting off hundreds of other conflicts, forming and severing friendships and sending multiple of governments into chaos. This event is the One Year War.
The One Year War -- also referred to as the Zeonic War for Independence -- takes place over 12 months (as you may have guessed), running from January 3 U.C. 0079 to January 1 U.C. 0080. This war is between Side 3 and the Earth Federation. Side 3 was a colony that was located near the moon. In U.C. 0052, Zeon Zum Deikun moved to this colony and quickly started to spread his philosophy of Contolism. Contolism was the idea that the Earth was sacred and that humans were destined to live in space. He also believed that all colonies should be independent of the Earth Federation. This culminated in U.C. 0058 when the people of Side 3 declared themselves independent and formed a republic.
However, tragedy struck in U.C. 0068 when Zeon Zum Deikun suddenly died, and his position was taken over by Degwin Zabi. Degwin quickly turned the republic into a dictatorship and declared that it was now the Principality of Zeon. On January 3 U.C. 0079, the Principality of Zeon declared war on Earth. Three seconds after the declaration of war, they attacked three colonies. Zeon used their Mobile Suits to completely obliterate the army on these colonies before murdering the civilians with poison gas in a heinous act that became known as the One Week Battle.
Zeon tried to destroy the Earth Federation's main base, Jaburo, but was thwarted. This caused Zeon to attempt to attack the colony of Loum, setting off a full-scale battle for the colony. This battle ended when Zeon used nuclear weapons to obliterate it. With nearly one-third of humanity dead, the Earth Federation debated surrendering to Zeon. However, they decided against this and instead made a treaty with Zeon banning nuclear and chemical weapons.
Zeon then invaded Earth, quickly taking over most of the Earth Federation's bases. During this, the Earth Federation attempted to build their own Mobile Suits. One of these attempts, dubbed Operation V, created the iconic RX-78-2 Gundam. On September 18, U.C, 0079, a Zeon craft discovered and attacked White Base, an Earth federation ship containing the RX-78-2 Gundam prototype. This battle led to Char Aznable and Amuro Ray having the first Gundam vs. Gundam fight.
On Earth, the Earth Federation began a major pushback, taking back several large bases during Operation Odessa. This marked the turning point of the war. In November, Zeon attempted to attack the federation base at Jaburo but was repelled by the Earth Federation. During this battle, Char Aznable and Amuro Ray once again crossed swords. Several other battles ensued with the Earth Federation slowly clawing back territory and holding off Zeon counterattacks. This culminated in the Earth Federation capturing the Zeon asteroid base called Solomon, destroying Zeon's Space Attack Force in the process.
Using this base, the Earth Federation began Operation Star One. This was an attempt to take the asteroid base A Baoa Qu. Degwin Zabi tried to go to the Earth Federation fleet, hoping to negotiate a peace treaty, but he was killed when his son, Gihren Zabi, fired a solar ray towards the fleet. Hitting Degwin's ship in the process. The Earth Federation continued to push and assaulted A Baoa Qu and the gathered Zeon forces. While this was happening, Gihren Zabi's half-sister, Kycilia Zabi had him executed for the crime of murdering their father. This caused chaos within the Zeon forces, many of whom fled the battle thinking that the war was lost. Zeon then surrendered to the Earth Federation on New Year's Day. Zeon was allowed to keep its independence as part of the treaty. However, some of the Zeon armies went into hiding, vowing to take over the Earth Federation in the future.
The One Year War devastated the galaxy, wiping out billions of people, destroying colonies and devastating massive swathes of Earth. These after-effects would cause a great deal of social and economic upheaval, changing life and causing problems for centuries to come. Many on Earth blamed those who lived in space for the war, causing a racial divide to form. However, the biggest effect of the war was the creation and normalization of Mobile Suits.
These hulking behemoths and their massive destructive power would become the standard for warfare going forward, making future conflicts even more dangerous and violent. The war's ending also left many former Zeon soldiers with a grudge and a lingering desire to get their revenge. This grudge would continue to grow through the years, causing instability and insurgency to surge throughout the galaxy, seemingly with no end.