1916: Einstein announces his general theory of relativity.

1997: Einstein again makes scientific headlines when astronomers announce signs that could prove the accuracy of the theory of relativity. Spinning bodies twist space and time along with them, which is an inevitable result of Einstein's theory.

1915: The Germans use chlorine gas at the Second Battle of Ypres—the first time poison gas is employed in conflict.

1961: The United States begins "operational field tests," spraying the defoliant Agent Orange over Vietnam. The program continues until 1971.

1995: Twelve people are killed in a terrorist attack using Sarin gas on a Japanese subway.

1999: The Defense Department begins a controversial program to vaccinate all military personnel against anthrax. Several soldiers who have refused on the grounds that not enough is known about the vaccine's safety have been court-martialed.