The Treaty of London, signed in 1839, provided international recognition for the newly formed state of Belgium. It was interpreted by the Allies as a guarantee of Belgian independence and neutrality (though Germany later rejected this):

A treaty between Great Britain, Austria, France, Prussia and Russia, on the one part, and Belgium, on the other. Signed at London, April 19th 1839…

Her Majesty the Queen of the United Kingdom of Great Britain and Ireland, His Majesty the Emperor of Austria, King of Hungary and Bohemia, His Majesty the King of the French, His Majesty the King of Prussia, and His Majesty the Emperor of all the Russias, taking into consideration, as well as His Majesty the King of the Belgians…

Article 1. The Belgian territory shall be composed of the provinces of South Brabant, Liege, Namur, Hainault, West Flanders, Antwerp and Limbourg…

[Articles 2-6 set down the territorial limits of Belgium, as well as its relationships with neighbouring states]

Article 7. Belgium, within the limits specified in Articles 1, 2 and 7, shall form an independent and perpetually neutral State. It shall be bound to observe such neutrality towards all other States…