The unique vulnerability of Seoul:

A conventional strike by the North is unlikely. It has used plausibly deniable asymmetric means to lash out at opponents for decades. But a Suleimani-style strike which targeted figures very close to the supreme leader Kim Jong Un – recall that Suleimani was number three in the Iranian hierarchy – could provoke something greater. And here South Korea’s unique vulnerability has always played a role in stopping U.S. and South Korean action against the North. Counter-strikes for the many North’s transgressions have always faltered because of the proximity (25 miles from the border), size (20 million people), and vulnerability of the Southern capital. It was too risky in the past, and that has not changed.