The Kandrian Empire had timed the application of this strategy quite well. It had used this tactic for the final and the most important territory in the Serevian Dungeon so that the element of surprise would be at the highest for the most important territory to be claimed.
The three nations did not mount a retaliatory force against the Kandrian Empire within the Serevian Dungeon in order to reclaim the core of the Serevian Dungeon.
Not because they chose not to, but because they couldn't, it wasn't viable. Each of the three nations had lost a large sum of assets to the branch attack that Rui had triggered. Their bases in the Serevian Dungeon were simply too lacking in the military and especially Martial Artist assets needed to siege and overrun the fortifications that the Kandrian Empire had erected around the core of the Serevian Dungeon that they had now claimed as Kandrian territory.