Defending South Korea

I was reading Michael O'Hanlon's policy paper on the scary question of what to do if a nuclear-armed regime collapses, and I was struck by this aside: "Pentagon planners have estimated the U.S. forces needed for the defense and ultimate liberation of the ROK to be roughly six ground combat divisions, including Marine and Army units, ten Air Force aircraft wings, and four to five Navy aircraft carrier battle groups – altogether totaling at least half a million Americans under arms."

How can that possibly be right? South Korea has twice the population of the DPRK and is far richer. In principle, the ROK ought to be able to defend itself adequately without any outside assistance. An American defense commitment to South Korea makes good sense (we get some influence in the region and it motivates South Korea to help us out with other stuff) even though I think they could get along without us, but there's just no way such an enormous quantity of assistance should be necessary especially because it would, in practice, take an unduly long time to move that much stuff to Korea in the event of a crisis. Meanwhile, South Korea has a $21 billion defense budget to North Korea's $5 billion and we're talking about helping the ROK with a defensive operation.

Something doesn't add up.