1600s: The Thirty Years' War rages in Europe from 1618 to 1648.

1930s-40s: World War II ravages Europe and the Pacific from 1939-45.

Today: There is no widespread warfare in the world. Global or continent-wide wars have given way to small pockets of geographically contained conflict such as the Persian Gulf War of the early-1990s.

1600s: The Thirty Years' War begins as a conflict of religious ideology, Catholic versus Protestant.

1930s-40s: World War II is a tactical war fought for geographic gain. A religious element still persists, however, in Nazi Germany's persecution of European Jews as well as other ethnic minorities who do not fit Adolf Hitler's Aryan ideal.

Today: Religion plays a role in several regional conflicts in the world. In Northern Ireland, it is Catholic versus Protestant; in the Middle East, it is Jewish versus Muslim.

1600s: Because the Thirty Years'...