"If you were asked when America became polarized, your answer would likely depend on your age: you might say during Barack Obama's presidency, or with the post- 9/11 war on terror, or the culture wars of the 1980s and 1990s, or the "Reagan Revolution" and the rise of the New Right. For leading historians Kevin M. Kruse and Julian E. Zelizer, it all starts in 1974. In that one year, the nation was rocked by one major event after another: the Watergate...