The Pinkertons were a company hired to break up strikes and unions by big business's.
The Pinkerton National Detective Agency (Pinkertons) was a private security company that businesses hired as strikebreakers or spies to bust or infiltrate labor unions.
They got big, and scary. At one point, there were more agents (/soldiers/police/mercenaries) employed by the Pinkertons than were in the US's standing army. The Homestead Strike of 1892 (Carnegie's steel mill) resulted in the Pinkertons going into a gunfight with labor union members.
The Pinkertons were responsible for the downfall of everybody's favorite cross-dressing murderous labor faction, the Molly Maguires.