Put a limit up to the last five decades, otherwise people will fairly say 1066 and stuff.
For me, on an acute but immeasurable way, it is 1987. The Hungerford Massacre changed rural society forever. Whereas a low crime rate (Hungerford had not had a murder since the 19th century) and thus a lack of surveillance would have been rightly boasted about before, all it meant is a lack of preparation that could be exploited.
Ryan was able to massacre so many people because of the parochial complacency, with many of the responding officers not actually knowing where Hungerford was.
It started future massacres (Dunblane, Cumbria, etc), and there are questions as to whether any long term lessons have been learned from it, as Derrick Bird did almost exactly the same thing in Cumbria, with a slow response again, and exploited the rural area, with police scrambling after him fairly ineffectively.