Oh they could have taken Pyongyang. It was going up towards the border with China, which didn't yet have troops in the fight, that was truly reckless. The war in Korea was not a full-fledged war; the comparison to the total war against Nazi Germany is not apt. It could be argued that MacArthur's attempting to "have his way" resulted in the current situation--a North Korea with nukes. It could also be argued that the Chinese weren't exactly eager to go into North Korea and if the U.S. had shown more restraint they wouldn't have. Furthermore, in the 20th century there are many examples of less than total victory: the Poles forced the Bolsheviks back after World War One but did not pursue them; the Western alliance did not occupy Germany at the end of WW I (a war in which MacArthur fought); George H.W. Bush forced Saddam Hussein out of Kuwait but did not overthrow him; Stalin did not overthrow the Finnish state after the Winter War but just took territory and demanded reparations; the Paraguayans did not seek total victory over Bolivia in the Chaco War--they just wanted them out of their country. Was the U.S. different in Korea because of its size? It had largely disarmed after WW II and had to scramble to prepare for even this limited action.