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, including 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 Ludlow Massacre was a watershed moment in American labor relations. Socialist historian Howard Zinn described it as "the culminating act of perhaps the most violent struggle between corporate power and laboring men in American history".[9] Congress responded to public outrage by directing the House Committee on Mines and Mining to investigate the events.[10] Its report, published in 1915, was influential in promoting child labor laws and an eight-hour work day.

Ludlow Massacre - Wikipedia
I don't think folks today realize what happened to create the protections currently being dismantled.