What does a ‘natural’ frontier mean?

Directions: Read the passage and answer the question.

The first of the political causes of war is war itself. Many wars have been fought, among other reasons, for the sake of seizing some strategically valuable piece of territory, or in order to secure a ‘natural’ frontier, that is to say, a frontier which is easy to defend and from which it is easy to launch attacks upon one’s neighbours. Purely military advantages are almost as highly praised by the rulers of nations as economic advantages. The possession of an army, navy and air force is itself a reason for going to a war. ‘We must use our forces now’, so runs the militarist’s argument, ‘in order that we may be in a position to use them to better effect next time.’