Colorado's most significant battles between labor and capital occurred primarily between miners and mine operators. In these battles the state government, with one clear exception, always took the side of the mine operators. Additional participants in Colorado's labor struggles have included the National Guard, often informally called the militia; private contractors such as the Pinkertons, Baldwin-Felts, and Thiel detective agencies; and various labor entities, employers' organizations such as the Mine Owners' Associations, and vigilante groups and employer-sponsored citizens groups, such as the Citizens' Alliance. The series of incidents that have most frequently been referred to as the Colorado Labor Wars involved a struggle between the Western Federation of Miners (WFM) and the mine operators, particularly the Cripple Creek Mine Owners' Association (CCMOA), during the period from 1903 to 1904. Like so many other fights between the miners and the owners of the mines, this was a brutal and bloody period in Colorado's history.