It was on this date in 1946, that the 400,000 bituminous coal miners of the United Mine Workers of America went on strike. It affected 26 states, and the strike continued through December. Their demands included safer conditions, health benefits and pay. This strike came at a time when the national economy was recovering from WWII, and Truman saw this as a negative for the ecnomy and approached the union with a settlement. The workers were not interested, and so they were fined $3.5 million, thus ending the strike. Although forced, most of their demands were covered in the compromise.
For more info:
http://www.foxbusiness.com/markets/2011/08/09/10-biggest-strikes-in-american-history/
http://shaybo-therisingtide.blogspot.com.tr/2012/04/april-1-1946-strike-by-400000-coal-mine.html