Well the problem with Korea was the fact that the US had that prick, MacArthur, in charge for the first half of it. If MacStupid hadn't have insisted that the UN Coalition keep marching northward, China wouldn't have got involved and we could have legitimately ended the war.
But, because of MacArthur, there is no way we could have actually 'finished' the Korean War.
China could not sustain it's fronts for any more than a few days, so they would attack and retreat, attack and retreat, et cetera. The UN Commanders didn't want to advance because we'd get destroyed -- the ROK forces had negligible effectiveness in combat (that's why earlier in the war, the DPRK forces easily disbanded the ROK forces, who always fought either side of UN forces, and then the DPRK could easily flank the UN forces).
The UN didn't want to lose more troops, the war was not popular in China or the US, but the Chinese and North Koreans didn't want the US-led coalition to get a favourable outcome to the war -- hence the reason why the Chinese-led peace discussions were so frustratingly drawn-out. Both sides stayed in the discussions because they knew that eventually the other side would crumble and give in to the other side's demands when they got fed up of losing troops.
As for what they're doing now? Well, I say let them build up their forces. It's a bluff. They're not going to do anything. Russia and China don't want to get involved in another war with the United States -- the world leaders are smarter than you'd think and none of them want to risk a nuclear confrontation. And North Korea knows that if it doesn't have the support of Russia and China, a UN-ROK coalition could roll right over the North Korean Army. Without Chinese support, NK's industry cannot -- I repeat, can not -- produce enough stuff (weapons, ammunition, planes, helicopters, light vehicles, tanks, fuel, etc.) to fight a war.