Why did both Korean governments claim authority over the entire peninsula?

Following the end of World War II and the liberation of Korea from Japanese colonial rule, the peninsula was temporarily divided along the 38th parallel by the United States and the Soviet Union. As Cold War tensions intensified, this administrative division hardened into two separate states in 1948: the Republic of Korea in the south and the Democratic People’s Republic of Korea in the north.123

Both governments claimed to be the sole legitimate authority over the entire peninsula because each viewed itself as the rightful successor to a unified Korean state, which had existed for over a thousand years prior to 1945. Consequently, both regimes formally asserted their legitimacy over the whole country and expressed the intent to reunify Korea, often threatening to do so by force. This fundamental disagreement over sovereignty and the desire for national unification were primary drivers of the Korean War, as both sides sought to establish their control across the entire territory.134