After the War of the Spanish Succession in the early 18th century, possession of the kingdom again changed hands. Under the terms of the Treaty of Utrecht in 1713, Naples was given to Charles VI, the Holy Roman Emperor. He also gained control of Sicily in 1720, but Austrian rule did not last long. Both Naples and Sicily were conquered by a Spanish army during the War of the Polish Succession in 1734, and Charles, Duke of Parma, a younger son of King Philip V of Spain was installed as King of Naples and Sicily from 1735. When Charles inherited the Spanish throne from his older half-brother in 1759, he left Naples and Sicily to his younger son, Ferdinand IV. Despite the two kingdoms being in a personal union under the House of Bourbon from 1735 onwards, they remained constitutionally separated.