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 is strategically very useful, as invading armies can have a hard time crossing such a border, and they may be easily defended.

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.