We unite because we must, and alliances such as these are fragile at best. A common enemy will force cooperation for only so long and only to such a degree. For the time being, the Charon are the only thing that keep up together.
The Galactic Emergency Coalition, also called the Alliance, was a massive, if loosly-maintained, ceasefire between nearly all of the relevant and surviving governmental powers of the galactic community in the interest of allying against the invading Charon Dominion.
As prescribed in the Treaty of Korriban, the galactic powers of the alliance were divided into four theaters; smaller alliances to operate independently of one another. The foremost was the Sith-led Dreshdae Concordant, which included also the Third Imperium and the Corporate Sector Authority. The Core Union was the next, which included the Chiss Ascendancy and the Jedi Order, and was spearheaded by the Galactic Federation.
The Mandalorian Clans were placed in charge of the Bloodmoon Pact, which allied them with the Hapes Consortium and their longtime enemies, the Echani Covenant. The final members of the alliance were the Hutt-controlled Godsheart Combine, which included with them the Gree Enclave and the remnant of the Centrality Confederation.