One of the endless problems of nationalism is finding low friction borders for nations. Oceans clearly work the best, but what else can be used?
At first glance, rivers look like they'd make good borders because they are line-shaped and they are moderately defensible in case of war. In reality, though, navigable rivers typically run through the heart of a nation (the Nile, the Mississippi, the Thames, the Volga, the Yellow, the Ganges, etc.). Indeed, nations are most likely to rise up along both banks of a mighty river.
Mountain ranges, such as the Pyrenees dividing France and Spain, seem more promising. They are military defensible and they reduce cultural and economic interchange, so the people living in the flat lands on either side of a big range tend to see themselves as different peoples.
The problem tends to be, however, that few mountain ranges are uninhabited. The mountaineers generally are often ornery folk who don't like being shoved around by flatlanders, and they live on both sides of the border. The Pashtuns who live on both sides of the Khyber Pass are the classic example. What could be a more logical place to draw the border between Afghanistan and Pakistan than the rugged mountains pierced by the famously narrow Khyber Pass? Yet, that logical border seems nonsensical to the millions of Pashtuns who live in the region and pay no attention to that line.