Great Galactic War

The Great Galactic war was a galactic conflict waged from 2612 to 2647. With every galactic nation involved and over two billion deaths, making it by far the largest and deadliest conflict in history. The conflict saw the last years of the Ur-Han Empire, the formation of the UCO and the emergence of the Terran Federation as the galaxy's dominant superpower. 
The conflict has been divided by modern historians into two main phases: the initial, first or early phase (2612-2636) and the Terran or second phase (2636-2647). The first phase was a traditional military confrontation between the Ur-Han Empire and the Surben Republic, the latter being aided by various political and separatist groups within the Ur-Han. This Surben-led alliance would come to be known as the Coalition. Among these groups was those known as the pretenders, aiming to install prince Derman on the Ur-Han throne. The other major groups were Tesrat and Dermkha separatists, who were often at odds with other coalition members. 
The second phase began with the Terran entry into the war in 2636. Although shorter than the first phase, it was much more deadly, with around 90% of casualties occurring in this period. Several Terran victories allowed the Coalition to achieve its objectives, and the initially anti-Ur-Han Coalition evolved into an anti-Terran Coalition. However the Terran incessant advance would only be halted by the Mirdesh entry into the war in 2640. In response, the Terran Federation began the large-scale use of WMDs during Operation Tenbatsu, largely against civilian targets so as to force favorable terms. This mass terror campaign caused an estimated 1,8 billion deaths, more than all other conflicts in history combined. The war remained a stalemate until late 2646 when the Terrans made a breakthrough in the Southern Theatre during Operation Invicta, which knocked out the Ur-Han out of the war with the Fall of Ur-Sumtka and ultimately led to the 2647 August Armistice, ending the war. 
The war has had a permanent impact on galactic politics. Though it made no territorial gains (the Dermkha Dominion is de jure not part of the Federation), the Terran Federation came out of the war strengthened relative to other nations. The Terran exit from isolationism has been described as "the single most important event in galactic history since the discovery of FTL" by Zarbaki scholar Juddum Larkhan. The creation of the Union of Cygnus-Orion has allowed for the Terran Federation to exert legislative influence on all other nations, with the ultimate aim of opening up their markets to Terran corporations. Most importantly, the Federation showed that its military was the most advanced in the galaxy, and that only a combined force of all other galactic powers could stop it. This has to an extent backfired, as other galactic nations began an extreme militarisation effort after the war, itself answered by intensive Terran militarisation. The total number of capital ships in service doubled during the war, and has quadrupled since. Nevertheless, the Federation retains its prime military position due to its technological edge and sole possession of supercarrier type ships. On the other hand, the traditional galactic powers of the Surben Republic and Ur-Han were utterly humiliated and reduced to a middling status. The Mirdesh Confederacy was the only traditional power to not be drastically weakened by the conflict, but its prestige was nevertheless greatly reduced.