The Shell Clan Civilization originally planned to withdraw their forces from Mushroom Star and focus all their efforts on fighting the Wandering Alliance.
They were well aware of their military strength and naturally did not wish to disperse their limited forces and be entangled in conflicts on two fronts.
However, what they had not anticipated was that as their forces on Mushroom Star began to withdraw, they were fiercely obstructed by the Mushroom Civilization.
