Complex sentence: Lithuanian border, which remained unchanged for about 500 years until World War I and partially survives as the border between the Republic of Lithuania and Kaliningrad Oblast, Russia, making it one of the most stable national borders in Europe.
Split into simpler sentences: Lithuanian border, which remained unchanged for about 500 years. A portion of the original border partially survives as the border between the Republic of Lithuania and Kaliningrad Oblast, Russia, making it one of the most stable national borders in Europe.