First Galactic War

The First Galactic War was a galaxy-wide conflict fought between the Imperium of Andarra and the New Republic that saw the creation of the Andarran demilitarised zone and the legitimisation of Mandalore as an astrapolitical entity. Fighting broke loose when the Imperium seized a number of Republican worlds in the Outer Rim, provoking the latter to declare war on the Imperium.

Relations between the Imperium and the New Republic
Ever since their first contact in the Corellian Skirmish, the Imperium and the Republic has maintained a hostile relationship with one another as they both saw each other as a menace to galactic peace. The Imperium view it as their duty to unite the galaxy under the banner of the Holy Mother and spread Her divine will—a prospect which the New Republic adamantly refused.

The July Plot
In 7.254.M17 A.D., a group of human supremacists in Concordia Dawn revolted against the New Republic on the basis that their planet had been tainted by the dirty hands of the Xenos. Their leader, Gilbert Ferdinand, secretly organised a coup d'etat against the local planetary governor, which he executed on the 9th of July. Though his revolution failed, it was powerful enough to send a message to the Imperium that the human population that lived in the New Republic are not content with their Xenos overlords.