Back in the 1930's Sweden was not the same as it is now. Economic depressions, instable governments as well as no social welfare programmes or workers rights meant that conditions were tough. Wage cuts, as well as scab workers that broke strikes and were protected by the military as local police didn't have the resources to guard them.
In protest against all of this, up to 4000 workers from several trades unions joined forces and formed a march in the central region of Dalarna, their target being the headquarters of the local strike breakers. The military protecting the strike breakers claimed that they heard shooting from within the ranks of the protesters and opened fire themselves, killing five and injuring the same number.
Reports about what actually happened that day vary according to who you believe, with different newspapers with different political affiliations telling different stories, but according to Roger Johansson from the University of Malmö, it was quite clear from the start how the events developed that day.
One myth from the right hand side of the political spectrum is that the workers attacked the military, he says, and that the military had to defend themselves. Another rumour was that the dead and injured had been hit by ricochets, but if you read the autopsies you can see that the majority were hit by direct shots.
The aftermath following the Ådalen riots changed Swedish history. Support for the left wing Social Democrats grew, employers became forced to negotiate, especially after the Social Democrats General Election victory in 1932. Eventually it lead to what would be known as the Swedish model, with unions and employers carrying out all discussions about employment regulations, and the government taking a back seat.
Another major change was that the military was banned from being used against civilians in peace time, a rule that also applies today, and was one of the reasons why the army wasn't called in during the riots in conjunction with the EU summit in Gothenburg a decade ago.