For example, in the US, there is still a lot of influence in how people view northern vs southern states after the civil war 150 years ago. In my state in the south, any time a federal election doesn't go our way, local politicians always bring up the idea of seceding again (but also while understanding it'll never actually happen).
Since America is only like 250 years old though, I'm curious if events from like 1000 or 2000 years ago still influence how people view each other or act within the country. How far back do you have to go before you'd say people stop being significantly influenced by events? Surely nobody in Italy treats other Italians differently because of something that happened in the early Roman empire, right?