On August 9, 1974—40 years ago this week—Richard Nixon stepped down as president of the United States in the wake of the Watergate scandal and, in the process, gave America a much-needed reality check.

Everyone makes mistakes. Even countries—our country—makes mistakes.

“America was beginning to think what it would mean to see itself as one more nation among other nations and one in which the same rules apply,” says Rick Perlstein, author of The Invisible Bridge: The Fall of Nixon and the Rise of Reagan. “And I think that was a healthy thing.”

In his new book, historian Rick Perlstein examines an America transformed by Watergate, and not entirely for the worse. In The Invisible Bridge, Perlstein tells the history of Watergate and years immediately after as an identity crisis between Nixon’s fallible America and Ronald Reagan’s mythic shining city on a hill...