At 4:00 a.m. Sunday morning, 25 June 1950, North Korea crossed the 38th Parallel (known as the "Line of Demarcation" in that part of the world), and launched a full-scale invasion of South Korea. The UN Security Council immediately passed a resolution calling on the "Democratic People’s Republic of Korea" (the DPRK) to cease hostilities and to withdraw.
When the North Koreans refused, the Security Council passed a second resolution on the 27th of June, recommending that UN members "furnish such assistance to the Republic of Korea [South Korea] as may be necessary to repel the armed attack and restore the international peace and security in the area."
Meanwhile, DPRK’s southern-moving forces were advancing rapidly. Seoul, South Korea’s capital, fell in four days.
These events posed a major challenge to the Truman Administration and to America’s allies. If the invasion were not checked, it would set a precedent that could easily undermine the confidence of post-WWII countries relying on the U.S. for protection. Countries like South Korea.
How was it that a nation, politically unified for two millennia, was now
fighting itself?