South Korea is understandably worried about the unstable kooks with nukes up north. We want to reassure them:
US Deputy Defence Secretary Ashton Carter on Monday promised to provide South Korea with every military resource under the US nuclear umbrella at a time of heightened tensions with North Korea.
Carter was in Seoul on the second leg of a four-nation tour of US allies and partners in Asia including Japan, the Philippines and Indonesia.
"We remain steadfast to our commitment to extended deterrence offered by the US nuclear umbrella," Carter said after talks with South Korean Defence Minister Kim Kwan-Jin.
We really have to nuke North Korea if North Korea uses a nuke on South Korea. If we don't, extended deterrence dies and no ally of ours will want to be without nukes of their own for primary deterrence.
Not that this means we have to incinerate a city if North Korea strikes a South Korean city. I think we could incinerate military bases with nukes and avoid mass murder, yet still maintain our credibility in extended deterrence.
But this just puts off the major problem of extended deterrence. The problem is that it only really works when we aren't vulnerable to nuclear strikes. Any Ally of ours has to worry whether an Enemy nuclear attack on our Ally will prompt us to use nukes on the Enemy if the Enemy can then threaten to fire a nuke at one of our cities.
That was always a worry of our allies during the Cold War. They worried if we would trade New York City for Bonn, Paris, or London, and worried we'd share interests with Russia in restricting a nuclear war to European territory. So Britain and France had enough nukes to rubble up Russia, which would prompt Russia to strike America, which would make America know we could not avoid general nuclear war by fighting just a limited nuclear war.
Really, at some point if nuclear proliferation among our enemies continues, it is in our interest to promote proliferation among our trusted allies so that they can deter--or fight--local nuclear wars without involving us. A blank nuclear check isn't something we want cashed when nutballs can fire nukes at us, too.