At end of WWII and the defeat of the Japanese in 1945 the Russians occupied the north and the Americans the south. Then followed the failure of Russia and America to agree upon a future for Korea and it was referred to the newly formed United Nations. Again nothing could be agreed except a boundary between north and south was settled on the 38th Parallel, with Russia and Communist China exercising control to the north and the Americans to the south. And in effect South Korea became a ward of the United Nations. As it remains today.
On June 27 1950 the fragile peace between North and South Korea was shattered when North Korean forces armed with Russian tanks and armaments swept, without any prior warning, across the border; the South Korean army melted away and had it not been for valiant holding actions by the US occupation forces in the extreme south the whole country would have been quickly over run. This action prompted the United Nations to call for an armed response, American President Truman authorized American forces to become engaged and shortly fifteen different countries promised military assistance. And among the first was Britain.