Galactic Terran Alliance
The Galactic Terran Alliance was an alliance formed by the human race after the unification of the peoples at Earth. Abbreviated as GTA. The space-based military wing of the GTA is known as the Galactic Terran Navy (GTN) but referenced as such only in the FreeSpace Reference Bible. The intelligence branch of the GTA is Galactic Terran Intelligence (GTI).
Speculation
Any info read here is not canon. The following section needs to be cleaned up to separate the facts from the interpretation.
The beginning
The Galactic Terran Alliance was formed in the first years of the age of space exploration, when the nations of the Earth have unified and formed an alliance. In the beginning, there were linguistic difficulties, but later, the English language later became a common language all through Terran space, even though not all GTA citizens spoke it as if it were their mothertongue, or did not speak it in a way that could be understood.
Initially, the alliance was focusing on economic and research factors, but later, with the discovery of subspace, building, maintaining and developing a space navy was at hand.
Expansion
The Alliance spread their empire all through the Sol system, building shipyards, repair yards and other kind of factories or supply depots. By the end of the 23rd century, the GTA had already taken up the whole Sol system and also spread to Delta Serpentis and Beta Aquilae.
Wars
In 2320, the Galactic Terran Alliance first encountered the Vasudans. At the beginning of their relationship, there was no armed conflict; what we know of as "The Great War" erupted in the year 2321. The exact cause of the war isn't revealed, but the FS1 techroom entry claims that one of the main factors was a Terran mishandling of "The Conversation", or a ritual of the Vasudan language.
After fourteen years of seemingly endless war, a previously unknown race known as the Shivans attacked, without warning, a Terran convoy and Vasudan strike force in the Antares system, as well as harassing other GTA and PVN forces in the sector. After initial casualty estimates, the PVN approached the GTA with a motion to sign an armistice and fight this new foe. Terran Command agreed, signed the armistice, and ended the Terran-Vasudan portion of the Great War. These two former foes would now cooperate in many ventures, such as science and engineering to defeat the newly discovered Shivans. With the signing of this treaty, the most important war in the history of either specie would now begin.
Ross 128 had already been lost, Betelgeuse, Beta Cygni and Ribos also fell to the Shivans' superiority. When the Shivans took Antares, it became clear to the GTA that if the Vasuda system were to fall, the Vasudans war effort would be fatally crushed. In the effort to defend Vasuda from the mighty Shivan fleet, multiple destroyers engaged Shivan forces in both Deneb and Antares. With the destruction of the GTD Galatea, nothing was left to halt the progression of Shivan forces into the Vasuda system. After arriving, the SD Lucifer bombarded Vasuda Prime, killing billions of innocent Vasudans after only fourteen hours of incessant bombardment.
With the destruction of Vasuda, Terran Command feared the Shivans would now move towards Earth. If it had not been for the Vasudan refugees who found the Ancient records, Earth would have also been annihilated by the Shivan superdestroyer. Thanks to the successful extraction of the Ancient data, Terran Command now possessed the knowledge needed to defeat the SD Lucifer and halt the Shivan advance. After being pursued by the GTD Bastion and later a small contingent of fighters and bombers, the SD Lucifer was destroyed in the subspace tunnel between Sol and Delta Serpentis, causing the jump node to collapse.
With the destruction of the SD Lucifer, the Shivan war machine crumbled, and the remnants of their fleet were defeated as the Great War finally came to an end.
Dissolution
Following the Great War and the loss of Earth, the GTA suddenly found itself without leadership. A provisional government was hastily formed from those members of the GTA government that were away from Sol when the Lucifer was destroyed. It barely held together a fragmented and despaired people, and the only symbol of its authority was the GTN, which was still licking its wounds after the Great War. TheHades Rebellion proved to be the catalyst for the collapse of the GTA. Even though the rebellion was defeated, the authority of the provisional government was in shambles, and humanity fell apart. The Galactic Terran Alliance was no more.
To fill the power vacuum created by the fall of the GTA, various independent nation-states were founded, such as the Adhara Coalition, the Antares Federation, the Luyten New Alliance, and the Regulus Syndicate. These various blocs were the only semblance of order in the anarchic post-Great War period of Terran history. Eventually, the Vasudans began overtures of a formal union between the two races, leading to the establishment of the Galactic Terran-Vasudan Alliance.
