The Secession War erupted between the UN and the Alliance of Free States.
The advanced technological research by the Alliance created a large gap in combat capabilities from the UN during their New Cold War. Suddenly, in 2111 the UN attacked first, and starting with a short and brutal phase with thousands of automated weapons destroying each other, war returned on an interplanetary scale. Satellites had been among the war's first casualties, and intelligence and reconnaissance relied on human pilots.
Population centers on Earth and space colonies were bombarded from space by particle beam weapons. Armies fought tank battles supported by buzzing swarms of aircraft. Starships battled each other over asteroid mines, orbital habitats and planetary installations. The furious pace of the war once again chewed up the stocks of men and materiel in a matter of months.
9 Bunker Hill-class light cruisers were built by the Alliance between 2111 and 2127
A major battle in the 2120s cost 95% of the forces involved in less than ten minutes. Sneak raids and lightning probes became the norm.
The Battle for Delta Pavonis V was fought in 2121 when the UN Geneva-class cruiser UNS Sophia came up with a innovative attack and destroyed the Allied heavy cruiser CVS Triphid and the three escorted cargo vessels.
The war progressed in violent fits and starts, punctuated by peaceful periods while the sides rebuilt their stocks. Civilian life was in little actual danger. In the rare and brief attacks, there was massive devastation. The balance of power shifted to the UN's favor, which had larger resource base and production capacity; the UN could outproduce the Alliance 10 to 1. The conflict turned into a war of attrition. The Alliance had the tactical edge, but it was being slowly beaten into the ground.
At some point the Alliance realized that they might have to deploy the experimental Hype/Telecon system which allowed human-machine control of real-time decision loop of automated combat systems; even if it was still an early prototype. This reversed the balance of power.
A new hope for the Alliance was the discovery of intelligent structures on the 6th planet of Deneb Kaitos, named Persephone. The USS Lexington and USS Jericho departed for Mission 161 to investigate but a spy notified UNS Dharma.
Original timeline
After the Battle of Persephone, the two crews annihilated each other and the sole survivor attempted to continue the investigation, the relief fleet of Midway, Iwo Jima and Otawa assisted by Concord and a squadron of destroyers arrives at Persephone and defeated the UN fleet, which in desperation, denied the installation to their enemies by "glassing" it with a nuclear bombardment.
In 2141 the Alliance signed the Washington Treaty and surrendered to the UN suffering a total defeat. The Treaty formalized the re-integration of former Alliance states into the UN.
Alternate timeline
With memories and information coming from the future, a crewman of the Lexington warned the captain about the ambush and using the Hype/Telecon system, he was able to defeat the Dharma.
UN Intelligence discovered that Hype was developed in a research facility in the Sierra Nevadas. For 2 months reconaissance pilots took off from UN bases deep in the South American jungles. Crossing the Pacific coast of North America at just over twice the speed of sound, threading valleys and passes at night to penetrate the Alliance regional air defenses. 3 UN pilots died before they had the imagery they needed.
The end of the war saw the Alliance winning with the Alliance Treaty making the dissolution of the UN as a formal political organization and spelling out the same political realities places UN member sates back to their old roles as Line One or Line Two nations.