A nation must have strong borders to survive. History is filled with nations that did not maintain control of their borders and died. Many borders over history have been geographical. That river over there. The big stone next to the forest. That mountain range. Other borders are imaginary lines in the sand either claimed by the nation or agreed upon with other nations. Many wars have started because nations couldn’t agree on where the border was, or because one of them just wanted that piece of land and the other one didn’t agree to let them have it. What does a nation do when people claiming they own your land cross your border without your approval and take up residence on your land? If you are a nation that wishes to survive, you send them back where they came from. By force if necessary. History is filled with nations that did not do that and died.