In the years following the defeat of Napoleon Bonaparte, his vanquishers redrewthe map of Europe, resulting in the creation of the United Kingdom of the Netherlands. It incorporated modern day Belgium and Holland.
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 the Treaty of London was signed on the 19th April 1839 by the major powers of Europe. This treaty guaranteed the independence of Belgium and Luxembourg, and the perpetual neutrality of Belgium. This latter clause was key 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.