Ethnic, cultural, and religious differences between the northern and southern territories resulted in the Belgian Revolution of 1830: a performance of the patriotic opera, La Muette de Portici, being the spark that lit the riotous fires. After an unsuccessful attempt by King William I to retake Brussels, a provisional government was established in the city in September 1830 and a declaration of independence was made in the following month.
Nevertheless, the country of Belgium was not recognised as an independent sovereign state until representatives of the major European powers signed the Treaty of London on the 19th April 1839. This treaty guaranteed the independence of Belgium and Luxembourg, and the perpetual neutrality of Belgium. This latter clause resulted in the declaration of war by the British against the Germans when the Kaiser’s army invaded Belgium in 1914.
The FirstWorldwar.com site has extracts of the Treaty of London available to read. As is the chapter of George Endmundson’s History of Holland, on the Authorama site, which focuses on the Belgian Rebellion.