President Trump, it seems, knows the price of everything and the value of nothing. Earlier this year he claimed that the U.S. gets “practically nothing” from the $1.2 billion it spends annually to keep 28,500 troops in South Korea. He forgets, or perhaps he never knew, that the U.S. waged an expensive and bloody struggle to restore the status quo ante after Kim Il Sung’s invasion of the south. American troops in South Korea have maintained peace and stability ever since—65 years and counting. Compared with the costs of war, it’s a bargain.
Mr....