If you’re old enough to remember the Soviet Union, you’ve probably wondered why so many young people today seem attracted to socialism. One influence is Howard Zinn, who published “A People’s History of the United States” in 1980, the year before the first millennials were born.

The book “continues to be assigned in countless college and high-school courses, but its commercial sales have remained strong as well,” the Chronicle of Higher Education reported in 2003, on the occasion of its millionth copy sold. It kept selling...