lets try to theory craft here
Let's say China believes the United States/South Korea is planning an attack on the North, and China/NK declares war on both countries.
What would be the expected outcome? Would the war stop on the first day? Who would win? What would happen?
there's a lot of implications that goes along with this scenario, the first of which being that china would declare war on a
presumption that the united states/rok would be preparing to conduct an assault. china is infamously known for being pragmatic in both bark and bite, especially with its greatest trade partner. in that scenario, I wouldn't expect china to declare war until there are foreign boots across the DMZ
assuming china preemptively declares war on the states, however, the united states has the ability to invoke article 5 of the nato constitution ("an attack on one is an attack on all")--thereby crippling the world economy and plunging the world not only into world war 3 but also into the second great depression. the world's two greatest economies (one a consumer, one a producer) and militaries would finally come to blows, surely inciting civil resistance in both theaters. the draft would most likely be invoked to negate the chinese 4:1 population advantage, and you can imagine the outcry regarding that kind of ridiculousness
assuming china preemptively declares war on the states and the states
don't invoke article 5, south korea and japan would still be called into war as per their constitutions (and some sections of seato, the asian equivalent of nato). china's superior geopolitical position would give it a humongous leg up in movement and attack--something witnessed during their first venture across the yalu in the original korean war. even back in the 50s, china's comparably inferior military pushed UN-US forces completely out of north korea and then comfortably held them right where they wanted them. with china's military equivalence to the states now, this would probably end up being the third instance of an atomic weapon being used in combat, although probably not by either of the giants; instead by china's lackey, north korea, who would at this point pepper seoul with all they've got to annihilate the 1/5 of the ROK's population that lives there and inflict some irreversible damage, even if china pulls out eventually because of the economic and diplomatic repercussion incurred by such a drastic war
you do have to remember that both the united states and china aren't stupid. they know conflict between them will only end in ruin not just between them but for the world. if any warring happens between them, it would be by air and limitedly by foot. no extended conflict would ever result
tl;dr ww3 second great depression
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