You can't build a real estate empire unless government helps you snatch private property, or so says Donald Trump, who routinely defends eminent domain as an inevitable business reality. Maybe he needs broader business experience. In Saturday's Republican debate, Mr. Trump fielded a question about eminent domain and had a ready answer. "You need eminent domain," Mr. Trump said. "A lot of the big conservatives that tell me how conservative they are," they "all want the Keystone Pipeline. The Keystone Pipeline, without eminent domain, it wouldn't go 10 feet, OK?" Not OK, or accurate. In the decade since TransCanada proposed...