The reason is very simple; the one who submerged... is the Head of State of the other country.
The Snow World Civilization itself is a small civilization with few strong figures. Because of this incident, it suffered heavy losses and was swiftly invaded by neighboring nations, engulfed in a bout of burning, killing, and plundering. Without any resistance, it was destroyed, the nation wiped out.
Afterward, that person was enslaved by the civilization that destroyed his homeland for over forty years, always enduring and accumulating Strength, until he finally raised his arm and unfurled the banner of reestablishment. Due to his immensely powerful Strength and an exceptionally intense desire for Revenge, the reestablishment process was somewhat fierce.
