NATURAL BORDER

A 'natural border' is a border between states which are composed of natural objects such as rivers, mountain ranges, or deserts.
Having a natural border is strategically very useful, as invading armies have a hard time crossing such a border. They are also easily defended.
Expanding until natural borders are reached, and maintaining those borders once conquered, have long been major foreign policy and war goals in the past. For example, the Roman Republic, and later, the Roman Empire expanded continuously until it reached its natural borders: First the Alps, later the Rhine river, the Danube river, the Sahara desert, and others.
From the Middle Ages onwards until the Industrial Revolution, France sought to expand its borders towards the Alps, the Pyrenees, and the Rhine River, while gradually releasing any territories held on the far side of those borders, because they were difficult to hold successfully in the long run.