North korea and china are allies and always have been. The North Koreans only have to worry about U.S. and South Korea. The united states has aircraft carriers and air bases in Japan, Guam, and South Korea, so if war broke out before the U.S could deploy any significant military forces of at least 750,000 troops in South Korea much like the UN Build up of the Gulf war 1 & 2, North Korea would reign well over 300,000 artillery shells and missiles on Seoul which is close to the DMZ.
The NK have a doctrine of huge blitzkrieg on SK before any meaningful American military is deployed on the ground. The NK have the largest special forces in the world in excess of 120,000 commandos trained to infiltrate the rear of the SK lines and reck havoc on supply lines, retreating SK troops, and SK reserves heading for the front.
The Americans have a large air assets in and around SK and Japan which the U.S. could rely on to bomb the NK in order to stem their advance. One must remember though that air power alone will not get the job done specially when limited to conventional air bombing. Within the first 24-48hrs the NK plan on capturing Seoul.
The U.S. cannot resort to nuclear weapons for 2 main reasons: The Chinese and Russians which share border with NK will not let America use nukes on their backyard which could have sever climate and health factors on their soil and secondly the NK would retailate with nuking SK, Japan, Alaska, Hawaii and California. If the Americans ignored the Russians and the Chinese the war could easily spread into something unimaginable and would pop the genie out of the bottle; and that is the use of nukes.
The NK with the huge infantry, armor, and special forces would easily overwhelm the 30k U.S troops and 300k SK troops on the border. The only way to seriously stem the NK advance and likely hood of the capture of SK is the use of nukes, and that is out of the question. If NK ever seemed to be losing, no matter how unlikely it is against SK, you better believe its a whole different ball game, the Chinese and Russians wont allow American troops into NK and at their doorstep. The will either work overtly or covertly to make sure America never captures NK.