You really have to explain how China kept those two divided. It seems to me you like to highlight China as the most important reason for the current mess and don't want to admit that it was USA and Soviet Union were the ones created it in the first place. Look at how you talk about your own country "US is also a factor". Not a factor but the one that was responsible in the first place along with the Russians. South Korea is also to be blamed for escalating the start of the Korean War.
Korean War - Wikipedia, the free encyclopedia
On 7 June 1950, Kim Il-sung called for an election in whole Korea on 5–8 August 1950 and a consultative conference in Haeju on 15–17 June 1950. On 11 June, the North sent three diplomats to the South, who were later arrested by the South. Fourteen days later on 25 June 1950, the North Korean People's Army (KPA) attacked South Korea.
The US intervened in the war instead of allowing North Korea to win against its southern part (thus to prevent unification) fearing the Communists would have too much influence in Asia and that Japan as the American trophy after WW2 could face harm (typical American paranoia). Thus the Americans intervened when things got out of hand between both Koreas. China only later intervened in the war in a reaction of American's involvement.
Don't try shifting the blame to China, the US were responsible for creating the mess. If you have proof that China had initiated the Korean War or have kept both Koreas away from unification after the end of the War do show us proof and not just empty words.