On this day...
some highlights from this article:

1919: Mexican anarchist revolutionary hero Emiliano Zapata, age 29, ambushed and assassinated by Mexican troops, Chinameca, Mexico. One of the main participants in the peasant uprisings against the central government's authority from 1910 until his death.

1945: U.S. medical staff at an Oak Ridge, Tennessee hospital inject plutonium into the survivor of a car accident. Thus begins an enormous (and until the 1990s, top-secret) U.S. government program, which did not end until the mid-1970s, to investigate the effects of radioactive materials when injected into live humans.

1947: Jackie Robinson appears in first game with the Brooklyn Dodgers, becoming the first African-American to play major league baseball after 78 years of segregation. The game, until a franchise moved to Atlanta in the mid-'60s, was played entirely in northern cities.

1981: United Nations approves world treaty assuring no civilians should be attacked with "napalm, mines, or booby-traps." Defeated by U.S. veto.