Seeing Russia invade Ukraine, historically neutral Finland has undergone a late conversion and decided to join NATO immediately. Why? Because NATO membership means the world's strongest power, the United States, under Article 5 of NATO, would go to war against Russia, should it cross Finland's border. Nervous about Russian President Vladimir Putin's intentions, Finland wants America legally and morally bound to fight Russia on its behalf, should Putin invade Finland as he invaded Ukraine. From the Finnish point of view, this is perfectly understandable. But why would the United States consent to go to war with Russia, the largest nuclear...