I take it you mean countries which are conterminous to one-another, since Spain and Germany are not neighbours and have no shared border. To the northeast of Spain is a severe mountain range called the Pyrenees, which separates it from France.

Physical features of the natural landscape have frequently played a part in determining territorial boundaries, when it was possible to reach agreement. Nobody can argue very effectively against the location of a mountain range or the course of a river.

In ancient Britain, the kingdom of Mercia was situated at one time in the area between the rivers Mersey and Trent - although subsequent warfare with neighbouring kingdoms caused expansion and contraction at times. Similarly, an early king of Powys was Elystan Glodrydd (meaning Elystan the renowned) whose territory was described as Rhwng Gwy a Hafren (meaning between the rivers Wye & Severn).