Actually, the destruction of the RWA (RRA is the Rift Republican Army; a splinter group which worked against the Amaris government) was due to many different factors.
I. Kerensky gutted the RWR's industry when he conquered the Republic. From what I can tell, some of the major industries like Rowe Weapons Systems survived on various worlds, but major industries like Krauss-Liemann and United Mechanics, LLC were shut down or converted to field SLDF materiel.
II. The SLDF hijacked the lion's share of the war materiel and burned for Terra, leaving the shaky Dormax government in power over a divided Republic. There were Dormax loyalists, RRA dissidents and Amaris sympathizers, all vying for power. This created an unstable civil war which drew the Lyrans into the conflict under false pretenses.
III. Kerensky never made good on his promise to liberate the Republic after the war. Robert Steiner simply conquered the RWR and never returned it, taking with it the best factories, including Weigel Armory & Munitions on Kwangjong-ni (later a Defiance Industries installation) and Blackstone on Inarcs (which made the advanced Talon before the war). This meant the surviving worlds would do without their signature weapons of war and could not sustain a military presence.
IV. Stefan Amaris abandoned the RWR militarily once Kerensky had shown his intentions to invade. Mohammed Selim had a scant 10 - 20 Regiments to defend the entire Republic, in addition to civilian militias and Castles Brian. Without the Republic's vast fleet, Kerensky easily picked apart these divisions and allowed some atrocities to be committed in the name of the SLDF. This included the destruction of entire regiments, execution-style.