China should simply say that as long as Western powers believe they have the right to interfere in other countries' affairs (known as regime change) and to establish bases to encircle China, it accords itself the right to aid an ally (albeit sometimes a recalcitrant one) at its border.  South Korea should be given the unequivocal message that they have a choice to either chose the Western camp or to, at the least, remain neutral, meaning get rid of foreign troops in the country.  In return South Korea will receive Chinese assurances that it will not stand in the way of eventual reunification, if South Korea will commit to the same policy vis-a-vis the reunification of Taiwan with China.

http://www.china-defense-mashup.com/n-korea-military-expansion-helped-by-china.html