Korea offers a useful lesson: Following the cessation of active hostilities there, U.S. forces remained on the ground in Korea. Today, they still remain, as a bulwark against North Korean aggression. If all American forces had left, it is entirely possible that North Korea would have by now launched offensives against the South. Countering the North Korean threat has cost the United States relatively little. Yes, our bases in South Korea cost money, and yes, American military personnel have been wounded or killed while on duty on the Korean peninsula. But we must weigh these losses against the cost of American inaction prior to the start of the Korean War — and this is where our history in Korea is particularly relevant to the crisis in Ukraine. We hesitated in our response to Communist aggression in Korea. We hesitated, and the South was overrun. The lesson is this: It is much easier to neutralize a threat by strongly opposing it than it is to expel an entrenched enemy. Si vis pacem, para bellum.