There is a lot involved in this, but here's the nuts and bolts.
When we bought our house 12 years ago, we knew that the neighboring house encroaches 8 feet onto our property. The survey we got when we bought our house clearly shows that.
Four years after we bought, our friendly neighbors sold their house to a jerk. For the last eight years we have co-existed, allowing him to park a car also encroaching on our property. We have been polite, said nothing, but have kept the survey pins marked and have mowed to the property line, right up to his house.
I came home from work after dark recently and could see that our property there was torn up by a contractor he had hired. I went out to locate the survey pins and my neighbor called from his porch, "Get off my property ------. Get the ---- off my property."
The next day his contractor helped me mark the property line. I told them I was in a great mood, and then was the time to settle this problem. Neighbor said there is no problem.
So, my buddy happen to be returning my jet ski trailer, and we put it there on my property, blocking the entrance to where the neighbor had been parking his car. I found the survey pin again, marked it, and set a large rock there. I marked the property line up near his house, and set a rock there (which has disappeared).
Next time I was mowing, my neighbor came charging out yelling at me to get off his property. When I got up close to his house, his woman dumped a pitcher of water on me from their porch, the portion encroaching on our lot.
That's in the front of his house. The same situation exists in the back, and he does not dispute that area.
So, what's a person to do?