The United States does seem to have a very violent labor history. I don't think I can answer your question that well because I'm still getting into the topic.

When the members of the American Railway Union decided to boycott the Pullman company in 1894 in response to the Panic of 1893, replaced labor called "strikebreakers" were called in order to fill the positions. When relations soured, President Cleveland filed an injunction to stop the boycott. I think riots took place as well.

But labor wasn't truly crushed until the 1920s as a result of the First Red Scare. Probably the largest case of labor violence took place in 1921 called the Battle of Blair Mountain, in which 10,000 striking coal miners and union workers fought 3,000 strikebreakers from Logan County. Basically, the United Mine Workers of America (UMWA) were deplored by the conditions of work, the subsistence wages given to the miners, and the wage slavery phenomenon called the company store. Logan County sheriffs would pay off detectives to bring back information of worker involvement with the UMWA.

So yes, I think labor unions have generally been targeted first. Some would even pay to make sure unions stayed out of their towns and cities. A lot of the violence perpetrated by the workers in that period were due to civil uprisings.

I can suggest "The Fall of the House of Labor" by David Montgomery. Also, this excerpt on American labor violence: http://www.ditext.com/taft/violence1.html http://en.wikipedia.org/wiki/Battle_of_Blair_Mountain http://dig.lib.niu.edu/gildedage/pullman/index.html