Complex sentence: Until the 17th century the peninsula was a chain of islands that formed a strip of land only during the summer; during the Battle of Hel in 1939, Polish forces dynamited the peninsula at one point, turning it into an island.
Split into simpler sentences: Until the 17th century the peninsula was a chain of islands that formed a strip of land only during the summer. In the course of the Battle of Hel in 1939, Polish forces dynamited the peninsula at one point, turning it into an island.