This thinking assumes that if North Korea attacked Seoul with artillery and missiles, it would inflict hundreds of thousands of casualties. But that isn't a given.
First off, at the tactical level, were the U.S. and South Korean militaries to anticipate an imminent North Korean attack on Seoul, their exigent mission would be the targeting and destruction of North Korean artillery systems. As a former South Korean Army general toldthe Korean Herald on Monday, the key to minimizing the potential destruction in Seoul is to strike both the Transporter Erector Launchers (TELs) that carry North Korean missiles, and their battlefield routes to the front line.
"The route is a fixed target like a tunnel or a bridge," said the general, "so we can detect and destroy it, then the TELs will end up at exposed launch sites. By concentrating on reconnaissance and airstrike assets, we can neutralize the remaining TELs."
This delivery of allied force would be overwhelming and, in terms of standard U.S. targeting processes, relatively indiscriminate. The intent would be the rapid destruction of North Korean fixed artillery positions and mobile missile launch capabilities, and in so the effective marginalization of the North's ability to inflict huge casualties on the South Korean population. Rather than worrying about killing too many North Koreans, the U.S. would seek to seize the strategic initiative with as great a decisive psychological impact as possible.
Moreover, the U.S. and South Korea have a heavy presence of counter-battery artillery and missile systems in proximity to North Korean offensive capabilities. And that presence isn't only about stopping civilian casualties in Seoul, it's about rapidly defeating any North Korean invasion.
By eliminating North Korean supporting fire for its ground incursion forces, the U.S. would compel Northern commanders to understand that their prospect for success has been eliminated. While the North Korean military is fanatical, the history of warfare proves that even the most resolute of combat forces are tempted to surrender when they sense a collapsing probability of success. The Nazis, for example, surrendered to the Red Army even though they expected it would mean their death.
In the same way, were their air power, fire support, and logistics trains all quickly isolated or annihilated, North Korean field commanders would have compelling reason to yield to reality rather than to death. Engulfed by fire and the prospect of imminent destruction, North Korean commanders would find throwing shells at Seoul to be a pointless gambit.
There's also the question of the strategic context here. Put simply, the strategic argument in favor of Seoul's safety from destruction is Pyongyang's existence. After all, were Kim Jong Un to order a major attack on Seoul, it would be an act of regime suicide on his part. In such a scenario, the U.S. and South Korea would be compelled to act to destroy the Kim dynasty, especially in the context of Kim's looming nuclear-ballistic missile threat to the U.S. mainland. Kim would have shown he is totally irrational and incapable of being deterred.
Fortunately, President Trump's comfort with having to potentially "totally destroy" North Korea helps the deterrent posture at this juncture. Like his father, Kim Jong Un is a voracious consumer of foreign news and attempts to understand Trump's mentality and seriousness. But based on Trump's aggressive use of force in response to Syrian ruler Bashar Assad's use of chemical weapons, the North Korean leadership must assess Trump's threats to be credible.
Correspondingly, Kim and his inner circle must ask the question, "what would an artillery/missile attack on Seoul get us?" While it might (might, again, being the operative word) kill tens of thousands of South Koreans, it would not allow the North to seize control over South Korea. But it would most certainly cost those leaders and their regime any chance of survival.
None of this is to suggest that Seoul is totally secure — it is not. Nevertheless, I do not believe North Korea's threat to Seoul is as great as commonly assumed.