Many years ago I read an article, I think in Scientific American, that the US could fight World War 3 with the then Soviet Union solely with cruise missiles, with conventional warheads, say a million, launched over a few hours. This was during the Cold War, and relied on the accuracy available at the time. Spy satellites, then and now, allow the US a clear picture of what is happening on the ground, although I know that there is scope for the observed nation to disguise its actions, or mislead the spy in the sky. Such an attack is a version of Blitzkrieg, or lightning war, long known to the military. The idea is to destroy enemy assets before they can be used.
Say ten thousand cruise missiles, aimed mostly at military targets in N Korea, launched within a few hours. If S Korea forbade such an arsenal on its soil, the US would need to move enough ships with all these cruise missiles off the coast of N Korea, which is not easy.
The Israeli military is already able to shoot down incoming short range missiles, such as Russian Scud missiles fired by Muslim groups. Such anti-missile missiles could be used against missiles with conventional warheads fired by North Korea against Seoul.
All war is risk, but such a blitzkrieg is tempting.