Thomas Jefferson’s Monticello is one of the most beloved sites in America, drawing more than 300,000 visitors a year up a steep mountain road to enjoy majestic views of the Virginia Piedmont and house tours that can feel like stepping into its creator’s complicated mind. But in 1775, it was a muddy construction site — and, as a guide told a tour group gathered on its front portico on a recent morning, a pretty good metaphor for the not-quite-born United States itself. “Things were just...