South Korea has the capability to build nuclear weapons very quickly if needed, they're a nuclear threshold state.

Nuclear weapons have a very brief transition from “everything is fine and nothing bad has happened” to “we’re completely fucked.” The fact that nothing has happened yet isn’t very reassuring to me with all the ways things can go wrong. The threat of retaliation certainly puts a damper on a first strike, but there’s always the possibility of a mistake, someone feeling backed into a corner, or not believing the consequences, or just going a little crazy. The more countries that have them, the more likely this becomes.