Nineteen men, women, and children participating in a strike are killed in the Ludlow Massacre during the Colorado Coalfield War.

The Ludlow Massacre was a mass killing perpetrated by anti-striker militia during the Colorado Coalfield War. Soldiers from the Colorado National Guard and private guards employed by Colorado Fuel and Iron Company (CF&I) attacked a tent colony of roughly 1,200 striking coal miners and their families in Ludlow, Colorado, on April 20, 1914. Approximately 21 people were killed, primarily miners' wives and children. John D. Rockefeller Jr. was a part-owner of CF&I who had recently appeared before a United States congressional hearing on the strikes, and he was widely blamed for having orchestrated the massacre.

The Colorado Coalfield War was a major labor uprising in the southern and central Colorado Front Range between September 1913 and December 1914. Striking began in late summer 1913, organized by the United Mine Workers of America (UMWA) against the Rockefeller-owned Colorado Fuel and Iron (CF&I) after years of deadly working conditions and low pay. The strike was marred by targeted and indiscriminate attacks from both strikers and individuals hired by CF&I to defend its property. Fighting was focused in the southern coal-mining counties of Las Animas and Huerfano, where the Colorado and Southern railroad passed through Trinidad and Walsenburg. It followed the 1912 Northern Colorado Coalfield Strikes.

April 20 is the 110th day of the year in the Gregorian calendar; 255 days remain until the end of the year.