The Ludlow Massacre was an attack by the Colorado National Guard on a tent colony of 1200 striking coal miners and their families at Ludlow, Colorado on April 20, 1914.

The Ludlow Massacre was a watershed moment in American labor relations. Historian Howard Zinn has described the Ludlow Massacre as "the culminating act of perhaps the most violent struggle between corporate power and laboring men in American history". Congress responded to public outcry by directing the House Committee on Mines and Mining to investigate the incident. Its report, published in 1915, was influential in promoting child labor laws and an eight-hour work day.

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