Historically, violence against unions has included attacks by detective and guard agencies, such as the Pinkertons, Baldwin Felts, Burns, or Thiel detective agencies; citizens groups, such as the Citizens' Alliance; company guards; police; national guard; or even the military.
Harry Wellington Laidler wrote a book in 1913 detailing how one of the largest union busters in the United States, Corporations Auxiliary Company, had a sales pitch offering the use of provocation and violence.