North Korea's extreme isolation is an intentional product of their government, not a consequence of US/SK politics.
The Korean War ended over half a century ago. There probably isn't barely anyone in NK left alive who remembers it. Hell, even Vietnam is in a far better spot presently and that country got the crap kicked out of it by everyone from the French to the Chinese for most of the last century.
Comparisons to Vietnam are frivolous and inaccurate. First off, N Vietnam won the war, took over the largely intact infrastructure in the south. Vietnam has a tropical climate with a year-round growing season and very large amounts of arable land, while N Korea has a miserable cold climate with a very short growing season and a very small amount of arable land... Nobody freezes to death in Vietnam in the winter, either... it's hard not to in NKorea.
None of which is to defend the NKorean regime in the slightest. Just because their govt is willfully isolationist doesn't mean we should reinforce their actions with ones of our own.
Successive US govts have believed that sanctions and isolation could force the collapse of regimes in Korea, Cuba, and Iran, none of which has happened or is likely to ever happen because of that. Maybe it's time we quit believing in our own lies... quit trying to use those peoples' misfortunes to further our own domestic political agendas.
