Complex sentence: Leopold's first war was the Second Northern War (1655--1660), in which King Charles X of Sweden tried to become King of Poland with the aid of allies including György II Rákóczi, Prince of Transylvania.
Split into simpler sentences: Leopold's first war was the Second Northern War (1655--1660). This war saw King Charles X of Sweden try to become King of Poland with the aid of allies including György II Rákóczi, Prince of Transylvania.