1923: The Soviet Union came into existence, expanding the Communist empire established by the Russian revolution of 1917.

1945: With the defeat of the Axis powers in World War II, the Soviet Union became one of the world's two superpowers, along with the United States.

1990: The Soviet Union disbanded after member nations demanded independence.

Today: Most former Soviet Union countries belong to the Commonwealth of Independent States: civil wars over property rights of ethnic factions break out frequently.

1923: Approximately 42 percent of the land in the United States was farmland; approximately 30 percent of the population lived on farms, down from 41 percent at the turn of the century.

1940: 46.8 percent of U.S. acreage was farmland; 23.2 percent of the population lived on farms.

1960: 49.5 percent of U.S. acreage was farmland; 8.7 percent of the population lived on farms.

1980: 44.8 percent of U.S. acreage was farmland: 2.7 percent of the population lived on...