Google Maps with the terrain feature activated enables one to see some of the world’s most naturally defined nation states.
Some interesting notes:
Iran is one of the most geographically well defined countries in the world. Tall mountain ranges delimit the Western, Northern, and Eastern borders. The ocean and desert mark the southern border. No wonder the core borders of Imperial Iran have changed little over the past several thousand years.
The Indian Subcontinent is also amazingly sharp: demarcated by ~1500 miles of Himalayas (India-China border) and Spīn Ghar Mountain range demarcating the Indian subcontinent’s border with Afghanistan and the rest of Central Asia.