Political units are often in part determined by geography. Oceans, rivers, and mountains all form natural boundaries/walls. An island is easily defined, and therefore typically exists as one political unit. Governments will make claim on land that has clear demarcation and has natural defensive properties. A mountain range or river offers natural protection or barrier from invasions. Castles will dominate a mountain landscape that is a border between nations. A river bank is easy to mark as property belonging to one political entity.
