The nature of the state includes its defense of national borders. Hundreds of years ago, city-states defended themselves with fortifications and armed guards, but now the cities have open borders and it is only the state that limits the movement of people. Historically, it can be seen as one of the aspects of the culture of war.
Civil society organizations often work to reduce the barriers with those in neighboring states. Here are examples from Cyprus, Hispaniola (Haiti and Dominican Republic), Northern Europe (Norway and Russia), Central America (Guatemala and Belize) and the Great Lakes region of Africa, among others.