If someone knocks at my door, I will examine them from a window, and decide whether I'll answer the door, or allow them in. If I don't want them in, the door will remain locked, and they will be asked to leave. If they don't, they become trespassers, and I will call the Sheriff. If they try to force the door, they will be stopped as they enter with a loaded gun. If they attempt to encroach further, deadly force will be used.
The description above is how most people in the United States react to a knock on the door. Some might not have a weapon, but will still notify authorities someone is attempting unauthorized access, and they need them to stop the invasion.
The border of the United States is the same. Access is by permission only. Attempting to forcibly enter is an invasion, and those involved are criminals.