The United Mine Workers of America begin a strike against the Rockefeller-owned Colorado Fuel & Iron Company, sparking the Colorado Coalfield War.

The United Mine Workers of America is a North American labor union best known for representing coal miners. Today, the Union also represents health care workers, truck drivers, manufacturing workers and public employees in the United States and Canada. Although its main focus has always been on workers and their rights, the UMW of today also advocates for better roads, schools, and universal health care. By 2014, coal mining had largely shifted to open pit mines in Wyoming, and there were only 60,000 active coal miners. The UMW was left with 35,000 members, of whom 20,000 were coal miners, chiefly in underground mines in Kentucky and West Virginia. However it was responsible for pensions and medical benefits for 40,000 retired miners, and for 50,000 spouses and dependents.