One of the more infamous incidents in the long Irish struggle for independence, the day that would become known as Bloody Sunday. There were actually three separate incidents over the course of the day.
The first was an operation by the Irish Republican Army (IRA) led by Michael Collins, that killed twelve British agents/informers and two Auxiliaries. In response, that afternoon, British military forces opened fire on the crowd at a football match in Croke Park, killing fourteen civilians (all Irish, of course). Finally, that evening, three IRA prisoners in Dublin Castle were beaten and killed by their British captors – allegedly while trying to escape.
The day became something of a rallying cry for both sides – both of whom would go on to further atrocities in the course of their long struggle with each other, a struggle that still goes on.
Sunday Bloody Sunday — U2