The Pinkerton Detective Agency used several aggressive tactics to bust unions during the late 19th and early 20th centuries:

• Infiltration and Espionage: Pinkerton agents infiltrated labor unions and organizations to gather intelligence, often acting as undercover operatives within unions. For example, detective James McParland infiltrated the Molly Maguires, a secretive Irish-American labor group, and provided evidence leading to arrests and executions of union members

• Providing Strikebreakers (Scabs) and Armed Guards: The Pinkertons supplied strikebreakers to replace striking workers and acted as armed guards to protect company property and strikebreakers. During the 1892 Homestead Strike, 300 Pinkerton agents were hired to guard the Carnegie Steel plant and break the strike, which led to violent clashes with workers

• Violence and Intimidation: Pinkerton agents used intimidation, physical violence, and sometimes lethal force against union members. Their presence often escalated conflicts, as seen in the Homestead Strike where a shooting war broke out between Pinkertons and strikers, resulting in deaths and injuries

• Collaboration with Authorities: After Pinkerton involvement, state militias or federal troops were often deployed to suppress strikes and restore order, further undermining union efforts

• Surveillance and Monitoring: Pinkertons monitored union activities and labor organizing efforts, including spying on workers to preempt unionization, a tactic still reported in modern times, such as Amazon hiring Pinkerton operatives to monitor warehouse workers' union activities

In summary, the Pinkertons used a combination of infiltration, espionage, strikebreaking, armed force, intimidation, and collaboration with state power to bust unions and suppress labor movements