if they were to launch a missile at all, it would be detected immediately by pretty much every surveillance system, and it would be thwarted either in mid air or by the time it gets low enough. let's hope it never comes to that though, and if we move troops in, we're able to disable their launching centers.
There are negotiations for the US to deploy the THAAD (Terminal High Altitude Area Defence) system in South Korea, but it's obviously opposed by nations, in this case Russia and China, since doing so would undermine their nuclear deterrent. The implication you're making that missile defence shields are standard is very much not the case. They would defeat the point of nuclear deterrent.
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