Kim Jong Un tested a new intercontinental ballistic missile in the early hours of Wednesday, and the data suggest it could hit all of the continental United States. If North Korea is allowed to perfect its warhead technology, it will be able to hold the U.S. hostage to nuclear ransom. The Trump Administration is right that the U.S. can’t live with this threat, so what more should it do to prevent it?

Conventional wisdom says that Pyongyang already faces extreme economic and diplomatic pressure. But in reality the United Nations...