William F. Buckley Jr. was the erudite heart of American conservatism. But the political vision that he helped forge was—and remains today—focused less on adhering to principles and more on ferreting out enemies.

Maybe the United States was doomed from the start. And Jean-Jacques Rousseau can explain why.

His anti-professionalism is part of a long populist tradition in America—and it could cripple his presidency.