1940s: Major cities in Europe and Japan suffer thousands of casualties in bombings during World War II.

1950s: Americans live in fear of a nuclear attack.

2001: Terrorists flying hijacked airplanes crash into the World Trade Center in New York City, into the Pentagon Building in Washington, D.C., and into the ground at another crash site, killing or wounding over 3,000 people. It is the first time the United States has suffered a large number of civilian casualties from attackers from outside the country.

1940s: CBS demonstrates color television in New York City, and WNBT, the first regularly operating television station, debuts in New York with an estimated 10,000 viewers.

1950s: Some 29 million American homes have television—approximately one in five. Most people still get their news from newspapers.

Today: Nearly every American home has at least one television, and most have two or more...