Complex sentence: Battle of Hel was one of the longest battles of the Polish September Campaign in 1939 and the Hel Peninsula, together with the town of Hel, was one of the longest-defended pockets of Polish Army resistance against the German invasion.
Split into simpler sentences: Battle of Hel was one of the longest battles of the Polish September Campaign in 1939. The Hel Peninsula, together with the town of Hel, was one of the longest-defended pockets of Polish Army resistance against the German invasion.