1939: Germany invades Poland and World War II begins.

Today: The conflict between the Israelis and Palestinians heightens tensions in the Middle East.

1941: On December 7, Japan attacks Pearl Harbor and the United States enters World War II. Today: Terrorist attacks on the World Trade Center towers in New York City and the Pentagon in Washington, D.C., on September 11, 2001, are the first large scale attacks on U.S. soil since the attack on Pearl Harbor.

1939: During the war years a strong underground movement emerges that helps stop Nazi aggression through sneak attacks, sabotage, and espionage.

Today: Espionage activities continue between the former Soviet Union and the United States, even after the Soviet Union abolishes its communist government. In recent years, spies have been caught and prosecuted in the United States, in Russia, and in China.