Beginning in September of 1913, 10,000 workers for the Colorado Fuel and Iron company went on strike to protest low pay and poor working conditions. Workers constructed a tent camp housing 1200 workers near the company town. On April 19th, 1914, the Colorado National Guard deployed a machine gun overlooking the camp and demanded that the strikers give up. On April 20th, they opened fire killing 25 people, 11 of whom were children.
I'm a lefty, like to think I'm fairly well-informed, but I'd never heard of this. Wow.