Complex sentence: The idea of the so-called three-isthmus border--defined by the White Isthmus, the Olonets Isthmus, and the Karelian Isthmus--is hundreds of years old, dating back to the period when Finland was part of Sweden.
Split into simpler sentences: The idea of the so-called three-isthmus border is hundreds of years old. It dates back to the period when Finland was part of Sweden.