While watching the Grammy awards last Sunday, it occurred to me that American culture has been defined by music ever since the end of World War II. After the Germans and Japanese surrendered in 1945, millions of GIs returned home to marry and begin families. The big-band era of good-time music accompanied that, and romantic singers like Frank Sinatra ruled the day.

In the ’50s, many young people began to rebel. The rise of Elvis Presley illuminated that rebellion. Then the angst kind of died out as Chubby Checker ushered in “The Twist” in 1960, and Americans began dancing all over the place.

Exhausted from doing “The Pony,” young consumers eventually began to respond to the snappy melodies of an English group called The Beatles, and once again, music mania gripped the nation. The British invasion featured the four mop-tops, the Rolling Stones and the Animals, among others.

Then came Vietnam.

That led to protest music and drug-fueled lyrics, as well as introspective tunes by the Doors, Jefferson Airplane and Bob Dylan. Acid rock soon followed, and everything was very far out, man.

After about seven years the dark themes receded and dancing came back. Disco took hold as the Bee Gees and other polyester-clad groups dominated the charts.

The late 1970s and early ’80s featured Madonna, Michael Jackson, and Earth, Wind and Fire. But it all ended when the AIDS scare arrived in 1984. Suddenly, the uninhibited party became dangerous.

Music kind of meandered for a while until rap emerged. At first, the anger-fueled recordings were confined to urban radio stations and a niche audience. But when Elton John sang a duet with the white rapper Eminem on a Grammy telecast, rap went mainstream. Massive parental headaches followed.

The rise of the Internet signaled the slow collapse of record stores, and the music industry quickly fragmented. Consumers could download songs onto portable machines and bop at will. Americans no longer had to depend on the radio to hear their favorite tunes.

Since then, there have been a series of pop superstars but no real purpose or point-of-view to the music, which again may reflect the current times. I mean, what do Lady Gaga and Jennifer Lopez really stand for? Narcissism? Just asking.

The talent is still there. I heard Justin Bieber do a knockout version of Paul McCartney’s classic “Let It Be.” And Bruno Mars with his little hat was pretty good on the Grammy show this year.

We are living in confusing, rapidly changing times. Fifty years ago, we were humming the same tunes heard over and over on AM radio. The good vibrations of the Beach Boys thrilled Maine, as well as Malibu. The music actually brought Americans together.

Today, the tuneless lure of cyber-space has pulled us apart. Perhaps forever.

Bill O’Reilly is host of the Fox News show “The O’Reilly Factor.”