Complex sentence: In June 1814, William I of the Netherlands, who was proclaimed Sovereign Prince of the United Kingdom of the Netherlands in 1813 and who would become King of the Netherlands on 16 March 1815, forbade Dutch participation in the slave trade by royal decree.
Split into simpler sentences: In June 1814, William I of the Netherlands was proclaimed Sovereign Prince of the United Kingdom of the Netherlands in 1813 and who would become King of the Netherlands on 16 March 1815. William forbade Dutch participation in the slave trade by royal decree.