The Interstellar Alliance was a governmental organization introduced in the Babylon 5 episode Rising Star.
A confederation of independent member worlds, the Alliance acted as a umbrella organization under which members could collaborate on trade, diplomacy, and mutual defense.
Following the last Shadow War, the various races that made up the former League of Non-Aligned Worlds came to the realization that they were stronger together than going their own ways. Following the end of the Earth Alliance Civil War and suicide of the EA's tyrannical President Clark the members of the League voted to disband, and joined the Minbari, Centauri, and Narn governments in forming a new Interstellar Alliance, with John Sheridan as its first President. Earth was quickly invited to join the alliance and soon accepted.
Members were asked to adhere to a declaration of principles that had been written by G'Kar and approved by the ISA council. Otherwise the day to day affairs were largely left to the member governments. In addition to the militaries of the member worlds, peace keeping in the ISA was handled by the Anla'Shok, which was opened to members from all alliance worlds.
Oppposed to the Alliance, the Drakh manipulated the Centauri into attacking the other members of the Alliance and started a war between the ISA and the Centauri. They then manipulated the Centauri into withdrawing from the ISA and going to war against them, an act which resulted in the devastation of the Centauri homeworld. The Centauri were later liberated and rejoined the Alliance when the Drakh connection was exposed by Vir Cotto.
The ISA also played a large role in curing the plague that the Drakh had infected Earth with, loaning their new Victory class destroyed Excalibur as a research ship to the Earth Alliance, and sending the Rangers to find clues to the cure.
Earth withdrew from the alliance in the 28th century after a devastating second civil war which saw its population reduced to a pre-industrial state. The ISA and Anla'Shok spent the next several hundred years working to bring Earth's people back to the stars.
A million years after its founding the ISA still existed. By then some of the founding members of the Alliance -including humans - were First Ones in their own right, having evolved beyond physical bodies and becoming beings of pure energy, much like the Vorlons before them.