From wiki: The Ludlow massacre pertains to the violent deaths of 45 people (32 of them women and children), during an attack by the Colorado National Guard on a tent colony of 1,200 striking coal miners and their families at Ludlow, Colorado in the U.S. on April 20, 1914. These deaths occurred after a day-long fight between strikers and the Guard. Two women, twelve children, six miners and union officials and one National Guardsman were killed. In response, the miners armed themselves and attacked dozens of mines, destroying property and engaging in several skirmishes with the Colorado National Guard.

This was the bloodiest event in the 14-month 1913-1914 southern Colorado Coal Strike. The strike was organized by the United Mine Workers of America (UMWA) against coal mining companies in Colorado. The three biggest mining companies were the Rockefeller family-owned Colorado Fuel & Iron Company (CF&I), the Rocky Mountain Fuel Company (RMF), and the Victor-American Fuel Company (VAF). Ludlow, located 12 miles (19 km) northwest of Trinidad, Colorado, is now a ghost town. The massacre site is owned by the UMWA, which erected a granite monument, in memory of the striking miners and their families who died that day.