The fall of the Old Empire wasn’t really over for the whole empiral era.
In fact, It was just the beging. A group of loyalist from the old Empire organized a strong militia of Mobile Suits and Ground forces along with a few dough up Mobile carriers and naval ships assaulted the western terr. of the New Republic. Because of the focuse of gorillawarfare and terrorist attack on the New Republic by Rebel offinceives, the New Republic had little military action over at the western terr. That soon brought up a major upset and a major victory to the rise of a new empire.

Soon, the new Empire of Zetsumei declared its command of that large portion of land that was once under control of the New Republic. This caused a sudden upriot and brought a 2 sided front war to the Republic.
However, the Empire was then confronted by the Rebels by a sudden ambush in the mid Pacific Ocean. The Empiral fleet of 45 some Mobile carriers and 36 navel ships along with over 450 Mobile suits where forced to retreat because heavy artillery and hit and runs by Rebels forces pounded their positions.
23 Mobile carriers made it back, 12 of which damaged. 12 navel carriers made it back, all of which however where badly damaged. Only 246 Mobile Suits where also lost in the battle.
This was also the Rebel’s 2nd major victory in the 2 sided front war.
And the New Empire’s 1st major defeat.