When America entered World War II, our society worked together with remarkable social cohesion, and the end of that war, which revealed the evil of the Holocaust, seemed to confirm the good we had done and the validity of American ideals.
During the 1950s, this cohesion seemed secure, although there was an ugly side to it. The House Un-American Activities Committee attempted to enforce conformity of thought by weeding out Communists, and fear-mongering became a technique used by some politicians with great success. But in the end, the Army-McCarthy hearings showed the excesses of this tendency, and the urge to apply the American principles of freedom, equality and self-government seemed secure.
Then came the civil rights movement, an extension of the urge to apply those ideals, and with is civil unrest. This was resolved through the democratic process as progressives from both major parties helped pass the Civil Rights Act and the Voting Rights Act.
The urge to spread the American way of life, however, led to an unpopular war in Viet Nam, and again, to civil unrest. Our draft laws tended to concentrate anti-war people of draft age in the universities, leading to student riots. To many Americans, it looked like the country was tearing itself apart. The Democratic Party, which had been ascendant since the 1930s, began tearing itself apart at its violent 1968 convention. Conservative Democrats began leaving the party and joining the Republicans, in part because of the Southern strategy of Richard Nixon.
George Packer, in his New Yorker article, The Fall of Conservatism, explored the history of this strategy. I recommend you follow the link and read the entire article. Here's an excerpt quoting Patrick Buchanan: