All of the nations had been surveilling the war at the heart of the Serevian Dungeon closely. It was an important war over the richest of territories with the highest esoteric resources yield. Whichever nation of the four nations won that war would likely 'win' the war of the Serevian Dungeon as a whole.
The Sekigahara Confederate, Republic of Gorteau, and the Britannian Empire had thought the Kandrian Empire had simply thrown in the tower as it chose not to send a powerful army, but instead a single Martial Apprentice. A single Martial Apprentice who, while no doubt extremely impressive, could not accomplish anything meaningful in a war of this scale all by himself.
Thus, one could imagine their sheer shock and bewilderment when a huge tide of branches swept across the warzone wiping out almost all of the military and Martial Artist assets from the entire battlefield.