Natural border

A natural border is a border between states or their subdivisions which is concomitant with natural formations such as rivers, mountain ranges, or deserts. The "doctrine of natural boundaries" developed in Western culture in the 17th century being based upon the "natural" ideas of Rousseau and developing concepts of nationalism. The similar concept in China developed earlier from natural zones of control.

Having a natural border has historically been strategically very useful, as invading armies could have a hard time crossing such a border, which could be more easily defended than 'regular' borders. Natural borders remain meaningful in modern warfare even though military technology and engineering have somewhat reduced their strategic value.

Expanding until natural borders are reached, and maintaining those borders once conquered, have been a major policy goal for a number of states. For example, the Roman Republic, and later, the Roman Empire expanded continuously until it reached certain natural borders: first the Alps, later the Rhine river, the Danube river and the Sahara desert. From the Middle Ages onwards until the 19th century, France sought to expand its borders towards the Alps, the Pyrenees, and the Rhine River.

Natural borders can be a source of territorial dispute when they shift. One such example is the Rio Grande, defining part of the border between the United States and Mexico, whose movement has led to multiple conflicts.

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